Early Head End Power (HEP)

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This page was last updated on September 10, 2023.

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(Portions of this article were first published in the UtahRails.net blog on December 9, 2011)


During the early part of the Twentieth century, most railroad passenger cars in the U.S. were heated by steam provided by the locomotive's boiler. If the cars had electric lighting, the power came from either a steam-driven generator in one of head-end cars, or from an axle-driven generator and was stored in large, heavy batteries.

The first electric lighting systems were tested on American railroads as early as the summer of 1882, when batteries were installed in Pennsylvania coach number 392. Batteries were found to be heavy, and hard to install and maintain properly, with short periods of use, typically 5-1/2 hours for batteries advertised as having discharge times of 7 hours. In 1887, Pennsylvania installed steam-driven generators in the baggage cars of two of its best trains, the Florida Special and the Chicago Limited. After these early installations, PRR continued to operate at least a few of its most important trains using electric lighting. Early installations of what were known as head-end plants were used successfully by Milwaukee Road in 1890 with the construction of special cars that were equipped with steam generators, to supply power to the road's 45 cars wired for electricity. By 1892, Milwaukee Road had 82 cars wired for electricity and ran five trains daily that were electrically lit. Hourly costs were reported to be ten times the cost of gas lighting. By 1904, Milwaukee had 300 cars wired for electrical lighting, but steam-driven generators and electrical lighting in general remained a novelty until about 1910. Electrical lighting was popular with passengers, but it was not practical or economic. One road kept it on its first class cars simply for its promotional value. (see John H. White, The American Railroad Passenger Car, pages 422 and 423)

In February 1906, Union Pacific announced that all mail cars were to be equipped with axle motors for illumination purposes, doing away with the dangers of exploding [Pintsch] gas. The electricity generated by the axle motors was to be stored in batteries with sufficient capacity to light the cars while the train is standing at stations. The change was being done in response to a request from the Postal Office Department, which had lost a large amount of mail due to gas explosions. (Ogden Standard, February 5, 1906)


By August 1908, steam-driven turbo-generators installed in baggage cars at the front (head-end) of the train, were considered to be the state of the art, and best system available to provide electricity. The early axle generators were small and produced limited power. These small axle generators provided power to on-board batteries, with were large, and also provided limited power.

To overcome these limitations and to obtain more capacity, in about 1903-1904, larger steam-driven dynamos were installed in baggage cars, with electrical wiring being run along the length of the train, usually in the form of conduits along the rooftops. Baggage cars were, in almost every case, operated at the front of the train, at the head-end, and the steam-driven dynamos soon took on the name of head-end power plants.

These head-end power plants used what were known as turbo-generators, which were steam-powered electrical generators intended for use in stationary power plants and other industrial applications where steam power needed to be converted to electrical power. The railroads were looking at having more electrical power available on their passenger trains, more power than axle generators of the period could supply. It likely seemed to be an easy solution to adapt these industrial steam dynamos for use on a moving passenger trains. The turbo-generators were big and heavy, and needed a steady supply of steam, so locating them in the baggage car (usually at the front of the train) was an obvious solution. A supply of steam was already available, since every passenger train made use of steam heat, using steam supplied by the locomotive at the front of every train. Electricity at the time was a developing combination of science and early materials engineering, and many of the components, such as switches and relays, required large panel installations. These control panels, along with the dynamos themselves, meant that a full-time attendant was needed to keep the equipment properly maintained and operating.

Union Pacific adopted the concept of head-end power plants in about 1905 with at least 15 wooden baggage cars being equipped with steam dynamos; these cars were labeled as Baggage Dynamo cars, and were considered by many to be cutting-edge technology. Several steel cars were delivered in 1909 and 1910, and in about 1914, the dynamos in the wooden cars were removed and installed in the steel cars. More steel cars were delivered in 1913, 1914, and 1921, making for an overall fleet of 63 baggage cars that used large steam dynamos to furnish electricity on the passenger trains of Union Pacific and its subsidiary companies.

Records are scarce, and research incomplete, but Jeff Cauthen has found by examining Pullman records, that in the earliest years of the Twentieth century, starting in 1905, E.H. Harriman's Common Standard Associated Lines (UP, SP and subsidiary companies), together with Chicago & North Western, received several wooden baggage cars specially equipped with steam-driven electrical generators, with the cars being labeled as "Baggage Dynamo" cars.

Those wooden baggage cars built in 1905 for Harriman's Associated companies were built on steel underframes, with steam-driven electrical generators. The cars were usually shown on diagram sheets and the pages in the equipment record book as Baggage Dynamo cars. Diagram sheets show what was called a "turbo-generator" installed in a separate compartment. The first wooden cars were delivered in 1905, with all-steel cars arriving in 1909 through 1914, with more arriving as late as 1921.

Wooden Cars

A total of seven dynamo cars were delivered in 1905. Four wooden 60' Baggage Dynamo cars were delivered from Pullman in 1905 as UP 1156-1159 (Pullman Lot 3265); renumbered to 900 series. (Photo of UP 1156 in SP Passenger Cars, Volume 3, page 211)

Three wooden 60' Baggage Dynamo cars were delivered from St. Louis Car Co. in 1905 as SPLA&SL 158, 160, 162; renumbered to LA&SL 4556-4558 in 1921-1922. The dynamos removed in 1924 and the cars were renumbered to LA&SL 4463-4465.

These seven cars were placed into service on the Los Angeles Limited, Union Pacific's premium, all Pullman sleeper train between Chicago and Los Angeles. The train was jointly operated by Chicago & North Western between Chicago and Omaha, by Union Pacific between Omaha and Ogden, and by San Pedro, Los Angeles & Salt Lake between Salt Lake City and Los Angeles (trains 7 and 8). The train used wooden cars until about 1912, and remained as the premier train until supplemented by the City of Los Angeles Streamliner in May 1936. The operation of The Los Angeles Limited ended in January 1954 when it was replaced by The Challenger.

(Read more about the Los Angeles Limited on Wikipedia)

(The portion of the run between Ogden and Salt Lake City was operated by Oregon Short Line as trains 17 (northbound) and 18 (southbound).

Three SPLA&SL Dynamo Baggage cars were delivered in June 1905 by St. Louis Car Company and numbered as SPLA&SL 158, 160 and 162. The three cars were renumbered to LA&SL 4556, 4557, 4558 in 1921-1922. In May 1924, they had their dynamos removed, and the three cars were converted to plain Baggage cars, then renumbered to LA&SL 4463, 4464, 4465, numbered after the six other Baggage cars delivered to SPLA&SL in June 1905, numbered as SPLA&SL 153, 154, 155, 157, 159, and 161. These six plain Baggage cars were renumbered to LA&SL 4454-4459 in 1921-1922.

SPLA&SL Mail 85, built in 1905, had a dynamo added in about 1910 and was renumbered to SPLA&SL 169, numbered after SPLA&SL Baggage Dynamo 166-168, built by Pullman in 1910. SPLA&SL 169 was likely renumbered to LA&SL 4562 in 1921-1922, and was retired before 1926, since it is not shown in the equipment record book.

The 900-series of dynamo baggage cars first appeared in the August 1910 Official Railway Equipment Record, without notation as to their construction, either wooden or steel. The same was true for the July 1911 issue, but the December 1911 issue separated the wooden cars (900-912, 916, 917; 15 cars) from the steel cars (913, 914, 915, 918-925; 11 cars).

UP wooden Baggage cars 1144-1155 (12 cars, plus one more car) may have had dynamos installed in about 1910 (or earlier), and were renumbered to UP 900-912.

Two of the original wooden dynamo baggage cars built in 1905, UP 1156 and 1158 were renumbered to UP 1163 and 1149 (dates not shown in equipment record book), indicating that their dynamos were removed, and likely installed in steel cars.

Although not yet documented, the two other wooden cars built in 1905, UP 1157 and 1159 were likely renumbered to UP 916, 917 in about 1910, when the Baggage Dynamo fleet was given its own number series.

Using the Official Railway Equipment Registers as a guide, at some time between the October 1908 issue and the May 1912 issue, Union Pacific put all of their Baggage Dynamo cars into their own 900-series number group. (More research is needed to better identify the date of the renumbering effort, with the ORER being published on a monthly basis.)

Modernizing The Fleet

With conversion of UP's premier trains from wooden cars to steel cars, the 15 wooden Baggage Dynamo cars were converted to plain Baggage cars, and their dynamos installed in newly delivered steel cars in the 100-series, which were then renumbered into the 900-series.

In the 1909-1914 era, there were 15 wooden Baggage Dynamo cars and 8 steel Baggage Dynamo cars, plus 3 wooden Baggage Dynamo cars on the SPLA&SL for trains between Salt Lake City and Los Angeles.

(Research question: What was used for trains operating over OSL tracks between UP at Ogden, and SPLA&SL at Salt Lake City? OSL received its first steel Baggage Dynamo cars in 1909, numbered as OSL 435-437.)

(Research question: What did ORR&N use for its premier trains equipped with electric lights? ORR&N received its first steel Baggage Dynamo cars in 1910, numbered as ORR&N 54-56)

Research indicates that there were a total of 15 wooden Baggage Dynamo cars, numbered as UP 900-912, 916, 917.

The Official Railway Equipment Register (ORER) for 1911 through early 1914 show a mix of wooden and steel Baggage Dynamo cars in the UP 900-series:

The same issues of ORER for 1911-1914 show the following 1100-series wooden and steel Baggage cars:

The ORER for January 1915 shows that the mix had changed, with the entire 900-925 group showing as steel cars, and the 1126-1150 group and 1155-1166 group showing as wooden cars.

The quantities from issues of ORER shown above indicate that at some point in the July to December 1914 time period, dynamos were removed from the 15 wooden cars in the 900-912, 916, 917 series, making them plain Baggage cars. The dynamos were then installed in the 15 steel cars of the UP 1155-1169 group, which were then renumbered to 903-912, 916, 917, 923-925. Additional research indicates that dynamos were acquired from other sources (possibly from wooden Baggage Dynamos 1156, 1158) and installed in three steel cars, UP 1148, 1149, 1150, which were then renumbered to UP 900, 901, 902.

Steel Cars (All Cars, as built and converted)

Car Number Previous
Type Qty Builder Date
1915 Car Numbers
LA&SL 4550-4555   Baggage Dynamo, 69' (steel) 6 Pullman Aug 1921   LA&SL 4550-4555 (1921)
OSL 435-437   Baggage Dynamo, 60' (steel) 3 Pullman Jul 1909   OSL 3100-3102
OSL 440-442   Baggage Dynamo, 60' (steel) 3 Pullman Mar 1910   OSL 3103-3105
OWRR&N 120, 121 OR&N 52, 53 Baggage Dynamo, 60' (steel) 2 Pullman Jul 1909   OWRR&N 1919, 1920
OWRR&N 122-124 OR&N 54-56 Baggage Dynamo, 60' (steel) 3 Pullman Feb 1910   OWRR&N 1921-1923
OWRR&N 126-131 (ORR&N 58-63) Baggage Dynamo, 60' (steel) 6 Pullman May 1911   OWRR&N 1924-1930
OWRR&N 132-136 (O&W 50-54) Baggage Dynamo, 60' (steel) 5 Pullman May 1911   OWRR&N 1931-1935
SPLA&SL 166-168   Baggage Dynamo, 60' (steel) 3 Pullman Jun 1910   LA&SL 4559-4561 (1921)
UP 900-912 UP 1148-1150, 1155-1164 Baggage Dynamo, 60' (steel) 13 Pullman 1909-1910 ca. 1914 UP 3000-3012
UP 913-915   Baggage Dynamo, 60' (steel) 3 Pullman Jun 1909   UP 3013-3015
UP 916, 917 UP 1165, 1166 Baggage Dynamo, 60' (steel) 2 Pullman 1910 ca. 1914 UP 3016, 3017
UP 918-922   Baggage Dynamo, 60' (steel) 5 Pullman Mar 1910   UP 3018-3022
UP 923-925 UP 1167-1169 Baggage Dynamo, 60' (steel) 3 Pullman 1910 ca. 1914 UP 3023-3025
UP 926-940   Baggage Dynamo, 70' (steel) 15 Pullman Dec 1914   UP 3026-3040
UP 950-954   Baggage Dynamo Buffet, 75' (steel) 5 Pullman Jan 1913   UP 2700-2704
UP 955, 956   Baggage Dynamo Buffet, 75' (steel) 2 Pullman Jan 1914   UP 2705, 2706
Total     79        

Dynamos from 18 900-series wooden cars were removed in about 1914, and re-installed in steel baggage cars in the 1100-series (UP 1148-1150, 1155-1169), which were then renumbered as 900-series cars (UP 900-912, 916, 917, 923-925).

(View a diagram of the OSL dynamo cars)

(View a diagram of the UP 60' dynamo cars)

(View a diagram of the UP 69' dynamo cars)

Steel Cars (As Built, By Year)

(18 converted cars not shown)

Date Built Car Number Previous Number Type Qty Builder 1915 Car Numbers
Jun 1909 UP 913-915   Baggage Dynamo, 60' (steel) 3 Pullman UP 3013-3015
Jul 1909 OSL 435-437   Baggage Dynamo, 60' (steel) 3 Pullman OSL 3100-3102
Jul 1909 ORR&N 120, 121 ORR&N 52, 53 Baggage Dynamo, 60' (steel) 2 Pullman OWRR&N 1919, 1920
Feb 1910 ORR&N 122-124 ORR&N 54-56 Baggage Dynamo, 60' (steel) 3 Pullman OWRR&N 1921-1923
Mar 1910 UP 918-922   Baggage Dynamo, 60' (steel) 5 Pullman UP 3018-3022
Mar 1910 OSL 440-442   Baggage Dynamo, 60' (steel) 3 Pullman OSL 3103-3105
Jun 1910 SPLA&SL 166-168   Baggage Dynamo, 60' (steel) 3 Pullman LA&SL 4559-4561 (1921)
May 1911 OWRR&N 126-131 (ORR&N 58-63) Baggage Dynamo, 60' (steel) 6 Pullman OWRR&N 1924-1930
May 1911 OWRR&N 132-136 (O&W 50-54) Baggage Dynamo, 60' (steel) 5 Pullman OWRR&N 1931-1935
Jan 1913 UP 950-954   Baggage Dynamo Buffet, 75' (steel) 5 Pullman UP 2700-2704
Jan 1914 UP 955, 956   Baggage Dynamo Buffet, 75' (steel) 2 Pullman UP 2705, 2706
Dec 1914 UP 926-940   Baggage Dynamo, 70' (steel) 15 Pullman UP 3026-3040
Aug 1921 LA&SL 4550-4555   Baggage Dynamo, 69' (steel) 6 Pullman LA&SL 4550-4555 (1921)
Total       61    

Conversion to Axle Generators

From about 1905 through to the mid 1920s, steam-driven dynamos in head-end baggage cars were the established method to provide electric lighting on passenger trains. Axle generators were first developed in the late 1880s, and the design for early axle generators continued to improve. These new designs allowed more powerful generators to be installed on individual cars on either the truck and wheel assemblies, or to the underbody of the passenger cars. These new designs improved the economics of axle lighting, making the cost of steam dynamo head-end plants a factor that needed to be addressed. These costs included the salary of a full-time attendant in each dynamo car.

Union Pacific interchanged numerous passenger trains every day with Southern Pacific at Ogden, Utah, operating over what is still known today as the Overland Route. Southern Pacific had adopted Baggage Dynamo cars at the same time as Union Pacific, because the two companies shared overall corporate ownership, and a shared concept of equipment design known as Common Standard.

(Southern Pacific's passenger car fleet has been thoroughly researched and published by the Southern Pacific Historical & Technical Society, in the form of a heavily illustrated, multi-volume history. This published history of Southern Pacific equipment shows that in the two decades between 1900 and 1920, SP history is much the same as Union Pacific's, and the SP society thankfully has chosen to include an unbelievable amount of Union Pacific equipment history in its publications.)

The history of Southern Pacific passenger cars shows that axle generators were included when SP began receiving steel cars in 1909. Earlier cars on SP (as well as on UP) had been equipped with gas lighting. On SP, gas lighting was replaced by axle lighting beginning as early as 1914. In 1924, Southern Pacific established its program to convert its trains with axle lighting, installing axle generators to replace steam dynamos installed in head-end baggage cars.

During the mid 1920s, UP continued to operate a fleet of twenty-six 60-foot Dynamo-Baggage cars, Class 60-DB, numbered as UP 3000-3025, and fifteen 69-foot Dynamo-Baggage cars (Class 69-DB), numbered as UP 3026-3040. In the equipment diagram books, the sheets for the 3000-series baggage cars appear to be identical in Union Pacific's 1926, 1930, and mid-1930s books, and show the cars as all being equipped with a dynamo, shown as a "turbo-generator," but the sheets do not break them down as to build date.

(View a diagram of the UP 60' dynamo cars)

(View a diagram of the UP 69' dynamo cars)

UP's Overland Route connection, Southern Pacific, converted its cars to axle generators in 1925-1926. In order to retain the ability to interchange cars used on through trains, Union Pacific likely also converted at least a limited number of its own cars to axle generators.

As mentioned, in 1924 SP adopted a change in axle lighting from truck-mounted generators to body-mounted generators, allowing for a larger, more efficient design of the generators themselves. These improvements in axle generators brought an end to the steam dynamos installed in head-end baggage cars.

Also in 1924, UP's Los Angeles & Salt Lake subsidiary modified its nine Baggage Dynamo cars by removing the dynamos, converting the cars to become full Baggage cars. Although documentation has not yet been identified, Union Pacific and its other two subsidiary railroads, Oregon Short Line and Oregon-Washington Railroad & Navigation Co., were closely tied together, making one think that at about the same time, they also began conversion to larger body-mounted axle generators, and removing their steam dynamos from the Baggage Dynamo cars.

Axle Generators

The amount of electrical power needed continued to increase as more lights were added throughout each train. Head-end systems were limited by the size of their steam dynamos, and their need for an attendant in the baggage car. There were also limitations of transmitting and controlling electrical power throughout the train. Axle-driven systems mounted to individual cars were better adapted to the increasing power needs. Truck-mounted belt-drive systems, mounted directly to the truck assemblies remained popular well into the latter half of the 1910-1920 decade, but body-mounted generators driven by axle-mounted flat belts gained in popularity as cost and reliability continued to improve. By the 1930s, existing heavyweight cars continued to use belt-driven, body-mounted generators, but the new lightweight cars began using an improved shaft drive and gearbox system connected the car's axle to a body-mounted generator.

Research in the U.S. patents has found some basic information about the first use of electricity on passenger trains, and its full adoption by 1910. Individual generators were attached to passenger car axles, with the design usually referred to as either "axle lighting," or "axle lights."

In 1899, axle-driven generators were installed on one hundred cars of the Santa Fe railroad. This was one of the first successful uses of a concept that was designed in Germany in 1882, and first tested in the U.S. in 1884. The earliest designs from the 1880s and early 1890s had to overcome problems such as how to keep the electrical current flowing as train speed increased and decreased as part of normal operations, along with current flow and polarity as trains reversed their direction of travel. Numerous designs were presented, and in June 1893, as discussed by John H. White in his The American Railroad Passenger Car, Morris Moskowitz was issued a patent for a design that transmitted electrical power from a passenger car's axles to a dynamo mounted inside the car itself. In a series of patents issued to Moskowitz in 1894 through 1897, the design of a 32-volt direct-current system was perfected that used a combination of axle-driven generators ("dynamos"), storage batteries, and control circuitry. Improvements continued and by mid 1904, a Moskowitz design (U. S. Patent 768392, August 23, 1904) emerged for an axle-driven generator similar to what was used in later years; a generator suspended from a bracket mounted to a railroad car's truck frame. A similar design was approved in March 1911 (U. S. Patent 986656, March 14, 1911) that further improved the idea of a truck mounted generator.

"It would be the work of United States Light and Heating Company, using Moskowitz's concepts, that led in 1902 to the NYC's premier all-Pullman train, The Twentieth Century Limited, to add axle lighting. With improvements and positive railroad responses, the number of electrically lighted cars soared, reaching approximately 15,000 by World War 1." (The Railroad: the life story of a technology, by H. Roger Grant, 2005, page 98)

A special conference was held in August 1908 to discuss the problems and practical solutions of electrical car lighting, and Morris Moskowitz was named as the eldest of the pioneers of "axle-lighting." The best method of car lighting was still an open question, including a consideration of design limitations of storage batteries. The report of a committee on head-end systems included this final remark about the use of a steam-driven dynamo installed in a baggage car at the train's head-end, "In the present state of the art, Mr. Gilman believes that the head-end baggage-car steam-driven unit presents the best system." A consideration of the future of axle lighting depended on a standardized axle design, and the committee determined to work with Master Car Builders on the matter of standardization of car axles.

Trade literature from the first decade of the twentieth century shows combination fixtures that had both electric lamps and gas mantles, for either acetylene or the Pintsch system. There are several examples shown in the 1906 Car Builder's Dictionary. I don't think these fixtures were a comment on the reliability of the electric systems, rather they allowed the cars to be used in service where a dynamo power supply wasn't provided. (Dennis Storzek, email dated November 15, 2011)

Axle-driven generators continued to be improved, with the associated reductions in cost of initial installation, which was less than head-end baggage car systems. Photos from 1910 show truck-mounted systems, but within ten years, the preferred design was for body-mounted generators connected to the axle by a flat-belt drive.

(U. S. Patent 1287519, December 10, 1918)

(U. S. Patent 1314757, September 2, 1919)

For the later shaft-driven, body-mounted systems, "A Spicer drive (the Spicer company was either bought by or became the Dana Corporation, famous for rear axle differentials on cars and trucks) was a shaft-driven drive for an underframe-mounted generator with a gearbox essentially like an automotive rear axle differential, except it was mounted directly on the wheel set axle. It dispensed with belts and the rubbing, friction, and wearing of related components when trucks had to swivel for a curve. But it wasn't the first shaft-drive mechanism for powering generators on rail cars. The "Pullman Drive", developed c1933 in conjunction with Pullman mechanical a/c, was a gearbox which worked like a Spicer, except that instead of being mounted on a wheel set axle, it was turned by pairs of pulleys mounted on a shorter gearbox axle, and drive belts between the gearbox axle pulleys and the wheel set axle pulleys. But these belts were always aligned and didn't suffer the degree of wear that the older belt drives experienced. Internal improvements to the gear train and differential were embodied by Pullman in c1935 and called a "5-B Drive", another mysterious example of Pullman naming. The "Spicer" drive was first adopted, I believe, in 1938 for the lightweight cars made for the NYC 20th Century Limited, although, in that batch of cars, both Spicer and 5-B drives were used, depending on the type of car." (John Friscella, Passenger Car List message no. 40778, dated December 26, 2005)

Some have wondered why body-mounted belt-driven generators were used for so long, several years after the first drive shaft axle-driven generators were invented by York Ice Machinery Company in 1931 for widespread use. Body-mounted belt-driven generators were used on the Santa Fe "Vista" series observation cars built in 1950, and on some economy baggage cars built in 1961, despite the problems of belt wear, breakage and friction. The best response would be that a flat-belt drive would be easier to repair in remote locations, "in the middle of nowhere," with just simple tools, provided you have a piece of spare belt. Not so with a Spicer drive, which was much harder to repair without special tools and specialized spare parts. (emails posted to Passenger Car List, November 14 and 15, 2011)

Air Conditioning

A review of data presented in William Kratville's "Passenger Car Catalog" published in 1968, pages 149-150, suggests that the first air conditioning (of any type) was installed in 1927, with numerous designs tried throughout the 1930-1935 time period. The three standard systems, ice activated, steam injector, and mechanical drive, came in 1933, 1932 and 1932, respectively.

Air conditioning was a major advance in passenger train design in the early 1930s. Passenger trains had previously been cooled by blowing air over ice contained in bunkers; it was the same system used in refrigerator cars on freight trains. It required supplies of ice en route, crews to load it into the cars before the prior load of ice had all melted away, and time in the schedule for the loading of the ice. The answer at the time was the steam ejector system, which was derived from the steam-jet system used to cool buildings. Pressurized steam sent through an orifice was used to create a vacuum in a cooling chamber, reducing the temperature inside the chamber and of a water supply which was then pumped to heat-exchanger coils in the air ducting, thus cooling the air circulated through the ducts. It was a complicated system that required constant care and maintenance but it eliminated the need for loading ice into the cars and was the best that the railroads had, until later advances in mechanical refrigeration. SP President Angus McDonald was insistent that his new Daylight train be the most modern streamliner in America, so it had to be air conditioned. The earlier Daylight trains used steam ejector air conditioning, but the 1941 train used Waukesha underbody air conditioners.

Eventually, the mechanical systems proved superior in performance and reliability. Southern Pacific converted its entire intercity passenger fleet to Waukesha air conditioning, so named for the manufacturer of the propane-powered engine/compressors that operated the system. Some railroads used axle drive to operate the compressors in their mechanical air conditioning, while still others used electricity generated by larger axle-driven generators and stored in larger batteries to run electric motors. Santa Fe stayed with the steam ejector system to its last orders for single-level streamlined cars, a decision which Santa Fe's then-president admitted was a mistake, especially after steam locomotives were replaced by diesels with steam boilers. The steam ejector air conditioning system was a prodigious consumer of steam and of water, requiring frequent stops to replenish the diesels' steam generator water supplies and the water tanks in the passenger cars. This even adversely affected Amtrak; in 1972, Santa Fe streamlined cars were transferred to Amtrak's Coast Starlight, whose route is primarily on Southern Pacific. But SP with its Waukesha air conditioning that needed no water to operate, didn't have watering facilities close together enough to supply water for the Santa Fe cars if trains ran late; the cars would run out of water, the air conditioning would shut down, and the passengers would have a hot, miserable ride until the next station that had watering facilities. Amtrak had to pay SP to install water hoses at several en route stations, and for SP crews at those stations to water the cars during extended station stops. (See Trainorders.com on May 25, 2011 for a discussion of air conditioning and lighting generators.)

By the 1930's air conditioning systems were light enough to install on passenger cars. Generally, diners were the first cars to be air conditioned as a measure of cleanliness against soot from the locomotive coming in open windows. Pullman had placed an experimental unit in one of its sleeper cars in 1929. The depression temporarily stalled any plans, but Pullman had been through depressions before and would be ready for the economic upturn. Initially only some cars were to be done. But when air conditioning became a selling point, everyone jumped on the bandwagon and whole trains were air-conditioned at one time. Pullman took advantage of their large pool of skilled workers to upgrade their huge investment. Thousands of cars were air conditioned during the 1930's, as well as hundreds that were rebuilt from older section-type sleepers into room-type cars.

Ammonia gas was used at first as the refrigerant, but only for a while because it was too dangerous. Freon came into use in 1932. These cars all had the Pullman mechanical type air conditioning, with a brine tank installed. The brine tanks were later bypassed, and most were removed.

(Read the Wikipedia article about Freon)

The installation of air conditioning in standard heavyweight cars changed a wider variety of structures on a car then often considered - from trucks to vents and everything in between. New insulation, new ductwork, new pipes, new roof covering, various modifications to trucks (generators, belts, etc.), re-arrangement of equipment (i.e. moving things around to gain room for condensers and compressors, and additional batteries, and then having to ensure a proper balance), new springing (due to increased weight), etc. etc. Some cars had few things moved, some had a lot moved, depended on the original configuration and the air conditioning suite installed. (Bob Webber, message to Trainorders.com, dated March 19, 2014)

Ice Activated Systems

When air conditioning was first used, blocks of ice were placed in bunkers located under each passenger car. Each car held about four tons of ice, preferable in blocks of 300-400 pounds each. As the ice melted, the cold/cool melt-water was pumped up to radiator cooling coils located in the roof area at the end of each car. Blowers moved air through the radiator coils and along rooftop ducts that were open to the passenger area. The blowers were operated from the car's batteries, which in turn, were charged by each car's axle generator when the car was moving. The water was recovered in a sump and sprayed over the blocks of ice to aid in their melting. Overflow from the sump was drained to the track. Known as "Ice activated" systems, this type of air conditioning began to be replaced in the high priority and premier trains when the steam injector system was developed. (Ice bunkers located in car roofs were used to load ice that was used only in the kitchens, and was not part of any car's air conditioning system.)

Some railroads used an interior duct system, while others, including Pullman, used an external duct.

The concept of ice water circulation for the purposes of air conditioning was in use as early as 1914, when Barney & Smith built Soo Line 991. The car is preserved at the Colfax Railroad Museum in Colfax, Wisconsin. (Herb Sakalaucks, via email dated April 12, 2013)

According to the 1942 Pullman Company air conditioning manual, "Ice Actuated" was the most reliable means of air conditioning a car. However, the car was not cooled by blowing air over the ice. Instead, a small circulating pump took the ice water that collected at the bottom of the ice bunkers and circulated it to collection of coils within the air ducting at the top of the car. The air was cooled by being passed through the coils. Some of the water was used to spray over the remaining ice in the bunker to create more iced water. Excess water was drained to the tracks.

Steam Injector "Ejector" Systems

Steam from the train line supply was piped into a cooling machine, where it expands, creating a vacuum which causes the water to boil at a very low temperature. This cools the water down to approximately 50 degrees Fahrenheit. The water was then pumped into the cooling coils in exactly the same way as an ice activated system.

Electro-Mechanical Systems

In 1935, the Waukesha Motor Company began production of its propane-fueled Ice Engine to provide air conditioning and electric power for railroad passenger cars and trains.

Waukesha made "Ice Engines" for air conditioning, and "Enginators" for lighting generators. Ice Engines all used a propane-fueled internal-combustion engine connected to an air conditioning compressor. The Enginators came in two basic styles. The propane version turned a DC generator for charging car batteries. They also made a diesel version but this unit turned a 240v 3 phase AC generator for cars that were equipped with AC electrical systems instead of DC. (Trainorders.com, October 10, 2005)

Waukesha Motor Company held several patents relating to its small internal combustion engine. The earliest patent for a complete engine came in January 1918, with numerous improvements throughout the 1920s. The design for a combined motor and transmission power unit came in December 1927 (U. S. Patent 1652432, December 13, 1927), and the design for a complete motor and compressor for air conditioning was patented in November 1937 (U. S. Patent 2099747, November 23, 1937). An improved engine and generator unit was patented in September 1949 (U. S. Patent 2482924, September 27, 1949).

The Waukesha Enginators were designed to start up when the power was getting low in the batteries or a large amount of power was needed. Instead of having a starter motor, the control circuitry would excite the generator to start the engine. This required quite a bit more power, so good set of batteries was needed. The Ice Engine had a starter motor, since there is no generator, and would start up with much less power. (Trainorders.com, October 11, 2005)

The early streamliners of the mid-1930's, such as the UP M-10000 and the Burlington's Zephyr, used electricity produced by propane-powered generators installed on the locomotive (head end power) for lighting and air conditioning. Almost all of the passenger cars built from the late 1930s through the mid-1960s continued to rely on steam for heat, and axle-driven generators for power.

With a few exceptions, most cars used axle generators, either belt driven, or shaft-driven by way of a Spicer drive on a truck axle. Since this was direct current, the polarity would be reversed as the car changed its direction of travel. General Electric made control relays that would insure that current retained its proper polarity. The Safety Company used a mechanical double-throw switch located at the end of the generator that engaged a polarity switch. When the direction of travel was reversed, the shaft rotated in the opposite direction, and the switch was thrown, changing the polarity.

Cars with axle driven generators used direct current power (usually 32, 64, or 110 volts DC). On-board features such as shaver outlets and audio systems needed AC power to work properly. In most cases, cars were equipped with motor-alternators. Using the classic California Zephyr trainset as an example, the cars were 32 volts DC. The motor-alternators had a 32 VDC motor driving a 110AC alternator. The 110AC was used for the train-wide PA system an lounge car radio, as well as all the fluorescent lighting.

Electricity for each car came from axle-driven generators and was stored in batteries. Heating was provided by steam from the locomotive running through radiators at floor level. Cooling was also from the pressurized steam line, through a system called "steam-ejector air conditioning."

The Waukesha Enginators, which were used in place of an axle-driven generator to keep storage batteries charged, were usually run year-round. The large propane tanks could potentially freeze up in cold weather. So, most under-car propane racks came equipped with a steam pipe that looped under each propane tank to prevent such an occurrence.

Intercity HEP

The first apparent use of head-end power for intercity trains were two semi-articulated trains built for New York, New Haven & Hartford Railroad between New York City and Boston in 1957.

The two New Haven trains were:

A third dedicated train for New York City-Boston service was New Haven's Budd RDC train "Roger Williams," placed in service in 1956. This train consisted of six RDC type cars coupled for multiple-unit operation, with each car being self-contained.

New York Central also attempted intercity HEP with its dedicated "Train X -Xplorer" between Cleveland and Cincinnati, Ohio in 1956. The train was noisy and had a rough ride and within a year was downgraded to commute service between Chicago and Elkhart, Indiana. Like the New Haven "Dan'l Webster" the NYC train's locomotive was built by Baldwin and the cars by Pullman, with slight variations in the designs.

The same designs for the Fairbanks-Morse locomotives and ACF cars were used by Boston & Maine Railroad for its "Speed Merchant" train in 1958.

All of these trains for New Haven, B&M, and NYC used the same 60Hz 440Vac head-end power.

Commuter Service HEP

During the post-war period, commuter trains became a means for workers to get from city centers out to the rapidly expanding suburbs, especially in the areas surrounding Chicago, San Francisco, and New York City. Depending on the railroad, some commuter trains continued to use steam locomotives, and some used newly delivered diesel locomotives, which were equipped with steam generators for heat, and locomotive-mounted generators for electrical lighting.

San Francisco

1955 -- Southern Pacific operated a fleet of 46 modern bi-level gallery HEP-equipped commuter coaches. SP received its first ten commuter coaches in June 1955. Built by Pullman-Standard, they were numbered as 3700-3709. A second group of 21 cars, numbered as SP 3710-3730, came from ACF in March 1957. The final group of 14 cars, numbered as SP 3731-3745, was delivered from Pullman-Standard in November 1968. (Southern Pacific Passenger Cars, Volume 1, Coaches and Chair Cars, pages 426-436, includes numerous photos and diagrams)


Between August 1950 and January 1951 the CB&Q received 30 stainless steel bi-level cars from Budd, also known as gallery cars. Although still equipped with steam heating, these are usually recognized as the first modern commuter cars in the Chicago area. C&NW commute service remained as steam-powered, with axle-driven generators to supply 32Vdc for lighting..

In May 1953, Chicago & North Western received new GP7s (C&NW 1650-1658) and June 1954 and September 1955, GP9s (C&NW 1711-1718) from General Motors' Electro-Motive Division. In addition to being equipped with steam generators, these locomotives were the first in the Chicago area with HEP for heating, lighting and air conditioning, and were equipped with auxiliary generators covered by special enclosures attached to the long-hood, driven directly from the locomotive air compressor, which itself was directly attached to the diesel engine. These generators produced 32Vdc electricity for lighting and 240Vac power for air conditioning. The change to 240Vdc HEP saved about five tons per car by eliminating the electrical generators and air conditioning equipment that was installed on passenger cars used in mainline service.

Additional need for 240Vdc head-end power in the mid 1950s, came from diesel generators placed in what were known as power cars, converted from heavyweight passenger cars. The 32Vdc for lighting also initially came from these same power cars. (These CB&Q cars were converted to all-electric 480Vac HEP in 1974, in a program that predated the Amtrak HEP program of 1976.) (CB&Q update from Thomas Cornillie via a posting to Railway Preservation News, March 4, 2015)

In 1955-1956, C&NW received 48 new streamlined, self-contained double-deck cars for suburban service. These first 48 cars were built with 7-1/2-KW Waukesha propane-fueled "Enginators" to provide 32Vdc electricity for lighting and 240Vac power for two underframe-mounted Trane E-M 8-ton air conditioning units. Locomotive-supplied steam was used for heating and some of these cars were pulled by steam locomotives.

In 1956, C&NW began substituting diesel locomotives for steam locomotives in suburban commuter service. To support of the change from steam power to diesel power, C&NW modified four 2250-horsepower E8 cab units by removing their steam generators and installing a skid-mounted A.C. power package consisting of a 575-horsepower Cummins diesel and a Marathon 480-volt, 60-cycle alternator rated at 300 kilowatts (with a 2-hour 10 per cent overload factor). The boiler water tanks were retained but filled with diesel oil, thus doubling each E8's fuel capacity from 1100 to 2200 gallons. Added together, the switch from steam to electric power in the rear of each unit boosted locomotive weight per E8 by 6200 pounds -- or to 337,200 pounds (168.6 tons), fully loaded. As protection power, C&NW similarly rebuilt a pair of 1500-horsepower F7 freight cab units. (Trains magazine, January 1959, page 17 and 18; Michael Palmieri, email dated November 22, 2011)

In 1959 C&NW changed from 240Vdc to 480Vac for its head-end power. C&NW gave four reasons for the switch from D.C. to A.C. power:

Concerning the question of where the 480Vac, three-phase standard for HEP came from, comments on the Railway Preservation Forum suggest that it may not have been the voltage itself, but the ready availability of reliable connectors from the maritime industry on the Great Lakes.

In January 1960, C&NW ordered 116 additional commuter cars, at a reported cost of $26.6 million, to be delivered through July 1961. These were the first push-pull cars in Chicago commuter service. This new bi-directional equipment and operation meant that the locomotive could remain at one end of the train, with engineer controls also being located in a specially equipped car at the opposite end. The locomotives were always on the "outbound" end of the train. This new design saved considerable shuffling of locomotives and cars at the Chicago stub-end passenger terminal. The regular double-deck "gallery" cars had 161 seats, while the control cab cars had 155 seats. After final delivery completed in mid 1961, C&NW would have 200 cars and 52 diesel locomotives assigned to its Chicago commuter service, carrying a weekday daily average of 40,000 passengers, on 157 weekday daily trips. These totals compare with the the totals from 1957, which were 432 cars and 63 locomotives. Of the current 157 daily trips, 33 were push-pull trains, using cars previously delivered to C&NW. (Trains magazine, August 1960, page 42) (Note that the operating voltage was not mentioned in the article.)

Milwaukee Road followed C&NW's example in 1961 with the delivery of 40 stainless steel double deck "gallery" cars that were also powered by head end generators installed in locomotives. (Trains magazine, August 1960, page 13)

By July 1961, C&NW was to have a fleet of 200 new cars and 52 locomotives modified for the new 480 Vac head-end power system. These replaced a steam-heated 32 Vdc fleet that in 1957 included 433 coaches and 63 locomotives. (Trains magazine, August 1960, page 42)


By 1968, the commuter trains in Montreal's GO Transit system had received its new GP40 locomotives, manufactured to include their own auxiliary diesel-alternator sets, supplying a new AC electrical system for heating, lighting and air conditioning, but with a different operating voltage, 575 volts compared to the 480 volts being used in the U.S. (Trains magazine, October 1968, page 24)

New York City

1971 -- Erie Lackawanna's new U34CH locomotives for commuter service in the New York area, were delivered in January 1971, and were essentially standard freight-service U36C's with an HEP generator powered off the prime mover, reducing the available horsepower from 3600 to 3400.

Amtrak HEP

Almost all of the cars acquired by Amtrak beginning in 1971 were steam-heated and had their own axle-driven generators. The arrival of 492 Amfleet cars between 1975 and 1977 began the wholesale change to HEP and allowed Amtrak to begin retiring older cars, especially coaches. While Amfleet cars came in several different interior configurations, most were 84-seat coaches intended for short- and medium-distance service, and none of them originally included sleeping accommodations. In 1976 Amtrak rebuilt 30 former U.S. Army kitchen cars into baggage cars 1350-1379, which could be used on either steam-heated or HEP trains.

(Read more about Amtrak's use of head end power, and its HEP program begun in 1977)

More Information

Passenger Car Mechanical Equipment Manuals -- PDF versions of obsolete service and repair manuals for several obsolete types of mechanical equipment used on passenger cars.