Union Pacific's DD35 Double Diesels

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By Don Strack

This page was last updated on March 19, 2022.

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(This article is an updated and expanded version of an article published in Diesel Era, Volume 12, Number 1, January/February 2001)


For much of the past 70 years, Union Pacific has been known for its large locomotives. During the steam era, there were the 4-12-2 Union Pacific types, the 4-6-6-4 Challengers, and of course the 4-8-8-4 Big Boys. During the diesel era, there were the 4,500 horsepower Standard and Veranda turbines, with their B+B-B+B truck arrangement, then the larger 8,500 horsepower, three-unit Big Blow turbines. In its later years, before the mergers began in 1982, there were the unique 6900-series Centennial units, with their 6,600 horsepower and just over 98 feet of total length. In between the Big Blow gas turbines and the Centennials, there were the DD35s and DDA35s, with their U50 and Century 855 stable mates.

These big locomotives were in response to UP's unique operating locations and conditions. In a motive power study completed in early 1962, the railroad found that the routine maintenance on any single diesel unit amounted to $7,000 per year, regardless of that unit's size or horsepower. This same study also indicated that approximately three horsepower per ton was needed to move each trailing ton of freight over UP's mainline trackage. This and other studies had been commissioned to develop a motive power plan that could see the railroad out of the turbine era and into its planned full embrace of the diesel-electric locomotives as its sole type of locomotive.

As the railroad moved out of the steam and turbine era, it found that rather than sizing each locomotive to be able to handle a train by itself, with an occasional helper being assigned, it could use the more efficient diesel locomotive, with its early small-per-unit horsepower, to operate multiple-unit consists that better match increasing train sizes. As freight schedules became tighter, and trains became longer, more and more diesel units were added. While the locomotive builders called this "flexibility," Union Pacific found that it was simply more expensive due to the constant annual cost per unit.

Road tests showed that 15,000 horsepower was needed to move UP's then-current fast freights, and that fuel capacity needed to be 1,000 gallons. Tests using a single 8,500 horsepower gas turbine along with a single 2,400 horsepower SD24 indicated that this 15,000 horsepower motive power set could keep a typical fast forwarder freight on its 41-hour schedule over the 1,530-mile route between North Platte, Nebraska, and Los Angeles, California, taking an average of 42.5 hours to make the trip.

With these test results in hand, UP approached the three major locomotive builders with a specification for a 15,000 horsepower, three-unit locomotive. Only one builder, the Electro-Motive Division of General Motors (EMD) was willing to build a prototype that matched the specification without an order. The other two builders, General Electric and Alco Products, were not willing to build a locomotive without a firm order. To expedite the testing of prototype locomotives from each builder, UP was forced to order three prototype units from these two builders, while EMD offered to build its units immediately as demonstrators.

The specification called for the prototype locomotives to ride on the twin B+B span bolster trucks from retired 4,500 horsepower Standard and Veranda gas turbines. This running gear, which had proven to be very successful for Union Pacific, consisted of two span bolsters, each with two motorized four-wheel trucks. The specification also called for each unit to be equipped with two separate diesel engines. Although the twin-engine concept had been used successfully by EMD since its earliest E-series passenger locomotives, UP stated that it was disappointed that the builders had yet to show an ability or a willingness to produce a single-engine locomotive in the 5,000 horsepower range. All current locomotives were using diesel engines in the 2,500 horsepower range, and UP said that all were simply upgrades from lesser horsepower designs, and that the upgrade concept was fast approaching its economical limit. Only the Swiss firm Sulzer had shown any ability to build a diesel engine in the horsepower range that UP wanted.

First Deliveries

The first of the prototype units built to UP's specification were completed in September 1963, but were the EMD DD35 demonstrators, which immediately began a nationwide tour. The first units to be delivered directly to UP came in October 1963, but not from EMD. These first three units for UP were the three U50s from GE. The three prototype units from Alco came nine months later in the form of two C-855 cab units and a single C-855 booster unit, and were delivered in June 1964.

The prototype units built by EMD were not to UP's exact specification of a three-unit, 15,000 horsepower locomotive. Rather, EMD chose to build a four-unit demonstrator set in the form of two 5,000-horsepower DD35 booster units, with two normal production 2,500-horsepower GP35s, one at each end. This set toured the nation's railroads in a striking red and white demonstrator paint scheme, with the tour ending on UP in May 1964. During its national tour, the set operated several times on UP rails, giving the road ample proof that EMD's design did indeed fulfill the road's initial specification.

This willingness by EMD to produce demonstrator locomotives without an order from UP, while both GE and Alco held out for firm orders, once again showed EMD's market-leading tactics. The company had taken the first-mover position for the manufacture and sale of diesel locomotives in the mid-1930s, and had maintained its position throughout the following 25 years. Alco had struggled with its own inadequate design and marketing efforts right from the first, as had Baldwin. Even after its electrical supplier, Westinghouse, took control of the company in July 1948, Baldwin still could not produce a reliable design. Westinghouse merged Lima (another locomotive company that used its products) with Baldwin in October 1950 hoping to gain a better foothold in the diesel locomotive market, but the venture failed and Westinghouse sold its interest back to Baldwin's shareholders in May 1954, having abandoned the locomotive market as a supplier of electrical gear the year before. Baldwin itself produced its last locomotive two years later, in 1956.

Union Pacific made note of the failure of Baldwin with both interest and disappointment, because it meant that one less builder was available as a potential, competing source of locomotives. Earlier, in 1945-47, the road had attempted to get Baldwin to deliver to UP a 6,000 horsepower, two-unit diesel locomotive to replace the road's 4-8-8-4 Big Boy steam locomotive. In October 1945, UP placed a firm order for what railfans today know as Baldwin's massive, 12-axle (2-D+D-2) Centipede locomotive. There were so many production delays that in April 1947, UP canceled the order and focused instead on multiple unit deliveries of much lower horsepower from both EMD and Alco to dieselize its freight operations.

This disappointment with designs from Baldwin was minor when compared with UP's eventual disappointment with the designs from Alco. Alco had delivered to UP a total of 44 Alco FA cab units and 44 FB booster units in 1947-48. There were sufficient problems with the reliability and maintenance costs of the Alco Model 244 diesel engine that when the time came in 1953 to buy additional large numbers of freight locomotives, UP turned again to EMD. Until the Century 855s of 1964, and after the cab units of 1948, UP only bought switching locomotives from Alco (discounting a few former demonstrator units, and some odd-ball units from other roads that themselves were merged with UP).

Alco went through many changes throughout the 1950s and early 1960s, seeking its place in a locomotive market dominated by EMD. In 1963, the company introduced its Century line, striving to prove that it was able to build a better, more reliable locomotive. Although the Century line was indeed an improvement over Alco's previous designs, the improvements still were not enough to satisfy potential buyers. UP's own experience with the Century line, with just the three C-855s in 1964, having been built to the road's 1963 specification for a three-unit, 15,000 horsepower locomotive, and just ten C630s in 1966, was enough to not generate any follow-on orders for either model. Both models were considered failures by UP and were retired from UP rails after just six years of service. Alco produced its last locomotive in 1969, after 121 years of producing locomotives.

By 1963, Baldwin had completely left the locomotive market, and Alco was making a last gallant try at regaining some of its glory from the steam era, but General Electric was really just getting started. GE had partnered with Alco from 1940-1953, furnishing electrical gear to Alco's designs. In 1953, Alco's inability to design and manufacture a reliable locomotive with reasonable maintenance costs, finally forced GE to end the shared relationship. Alco had been taking advantage of GE's much larger marketing organization, but GE felt that Alco's inadequate designs were now pulling GE's own reputation down to unacceptably low levels. While serving as the marketing focal point for the joint Alco-GE locomotives, GE had learned that there was indeed a place for a well-designed and reliable locomotive, and soon pursued its own designs. Between 1954 and 1959, GE methodically worked at gleaning the best features from several test designs, and took the market by storm with its ground-breaking U25B unit in 1960. UP itself bought a total of 16 U25Bs. The units that GE furnished to UP to match the road's specification for a 15,000 horsepower, three-unit locomotive, were designated by GE as U50s, and were essentially two U25Bs mounted atop a common frame, running on the retired gas turbine running gear as called out by UP in its specification. The fact that the three initial U50s would lead to a follow-on order from UP for another 20 units in June 1964 to August 1965 speaks largely of GE's commitment to improving its product line, something that Alco was unable to do. UP's 23 U50s remained in service for a total of 14 years, seeing service mainly on UP's eastern, mostly flat mainlines.

Although EMD was the subject a federal anti-trust investigation from September 1961 to December 1964, for its domination of the nation's locomotive market, it continued to develop new designs, or at least upgraded and improved designs. As the company moved from the turbocharged SD24 and GP20 models, to the innovative GP30, to that unit's successor, the GP35, EMD continued to be the market leader. And the company's willingness to build a prototype 5,000 horsepower, single-unit locomotive matching UP's newly released specification showed that it was again on the cutting edge of locomotive development.

EMD's DD35s

To meet UP's design specification, EMD's design, the DD35, was announced in the May 20, 1963 issue of the trade publication Railway Age. It would consist of two 16-cylinder 2,500 horsepower diesel engines mounted on a single locomotive frame, and was actually (with the exception of the trucks) all of the mechanical and electrical components of two of EMD's current GP35 2,500 horsepower locomotive combined into a single unit, including fuel tanks. The truck design was a four-axle version of the successful Flexicoil three-axle design used on EMD's SD24 locomotive, of which UP owned 30 cab units and 45 cabless booster units. This new truck design was cause for concern by UP, and the road stated that it would watch the performance of the four-unit (two GP35 cab units and two cabless DD35 booster units) demonstrator with great interest, looking for potential design and maintenance problems on the new truck.

The four-unit demonstrator set was completed in September 1963 and immediately set out on a nationwide tour that included extended tests on UP. Union Pacific was so pleased with the design that they almost immediately ordered an additional 25 DD35s and 22 matching GP35s, delivered in May to August 1964. The road made formal purchase (at a reduced price) of the four unit demonstrator set in May 1964. (Southern Pacific was also impressed with the demonstrator set and bought three DD35s for its own operations, keeping them in service until 1979.)

As with UP's earlier experiences with cabless booster units, the lack of flexibility of these massive, mainline-only units soon became apparent, and a planned follow-on order for 15 units was changed from booster units to 15 units with fully equipped cabs. The 27 DD35s were Union Pacific's last cabless booster units, and all new units since then have come equipped with cabs. The 15 new cab-equipped units were designated as DDA35s and were delivered in April to June 1965.

In the design for the new DDA35, UP stipulated that the new cab-equipped units not be any longer than the previous cabless units. This forced EMD to move the forward diesel engine to the rear to give space for the cab. This rearrangement of components included raised and shortened radiators, which gave the cab units their distinctive flared radiator sections. Besides being 5,000 horsepower DD units, the cab units were somewhat unique on UP in that they were completed using trade-in GE traction motors from retired Alco FA cab units and FB booster units, except for UP 84, which was built using the trade-in traction motors from wrecked UP GP9 159.

There were reports by retired employees that the problem was that when UP asked for a DD35 with a cab, EMD wanted more money to make a longer DDA35 frame. To save costs, and use the same jigs and fixtures from the DD35 frame, and still have space for a cab, the DDA35 components had to be pushed together inside the car body to fit on the DD35 frame. They also said that the radiators were shorter but wider to keep the same square inches of metal.

But a quick look in the EMD parts catalog shows the DD35 and the DDA35 both with six sections of radiator cores, which were the standard EMD length of 27-21/32 inches. The question is: If they both used six sections of 27-inch cores, why were the DDA35 cores wider. Both locomotives were 5000 horsepower, and needed the same cooling capacity, and the parts catalog shows that they were the same length. The answer was that although they used the same cores, they used different headers and piping between and among the cores, all of which was jammed into the shorter car body space.

The total fleet of 27 DD35 booster units and 15 DDA35 cab units entered service on UP pulling the road's cross-country fast freights. Unlike the SD40s that came just a year after the DDA35s, these eight-axle units remained almost solely on UP rails (there are reports that at least one unit made it as far east as Jacksonville, Florida, in pool service with Seaboard), while many of the road's newer six-axle units wandered onto several connecting roads in pool service. In fact, the SD40s were purchased specifically to allow them to be used in run-through pool service with connecting roads. UP had offered to allow its much larger units to be used in pool service, but the connecting roads were quite emphatic that they were not in the least interested in the potential rail damage from the units' long wheel base Flexicoil trucks.

IDAC and Genisco Wheelslip Control

The DD35s (and accompanying GP35s) were the last units from EMD using the DC generator. While these units were the newest from EMD at the time, modern electronic wheelslip control did not yet exist. With the later SD40s, and their newer AC traction alternators, came better wheelslip control, and UP soon began looking at upgrading the DD35s to allow them to remain useful in their place in fast mainline freight service right alongside the newer SD40s, and still newer DDA40Xs. To match the DDA40Xs, with their all-electronic controls, UP began a program to upgrade the SD40s with EMD's IDAC wheelslip control. Unfortunately, the IDAC system was suitable only for AC locomotive systems, so UP began a program to design a similar system for its newer DC locomotives, the DD35s and the GP35s. In concert with a company by the name of Genisco, UP was able to get a system that performed comparably with EMD's IDAC system. Genisco also had some success marketing its system to other roads with similar locomotives in similar types of service.

Radio Control Operations

UP 82 and 82B were equipped with Locotrol radio control systems (RCS) in early 1968 as part of a test of radio control operations on UP. The two DD units were selected to compare their characteristics with those of the first two SD45s equipped at the same time, UP 3622 and 3623. While no additional DD units were equipped, UP ultimately equipped 19 SD45 even-numbered as RCS masters and 18 SD45 odd-numbered RCS remotes.

UP and AT&SF were on the forefront of the development of RCS operations, AT&SF having first used it on its York Canyon coal trains in 1967, which operated from York Canyon in New Mexico, to the Kaiser steel mill at Fontana, California. Today known as distributed power, radio control was being developed under the trade name of Locotrol, which placed radio-controlled mid train helpers further back in the train on high tonnage trains, with all locomotives under the control of the train's single engineer. First designed in 1963, and tested on Southern and Kansas City Southern in 1965 and 1966, the first use by western railroads was when Santa Fe installed it in 1967 on York Canyon coal trains. UP completed its first installations of RCS in 1968.

Like the SD45s, UP 82's installation of the RCS radio equipment was in the unit's short hood. For the 82B, it was installed at the center of the unit, on the interior wall of the center crossover walkway. The initial antenna configuration consisted of a pair of "firecracker" antennas mounted directly on the locomotive cab roof on the 82, and on the roof over the center walkway on the 82B. Difficulties in radio signal reception resulted in a redesign of the antenna system to include an antenna platform, or ground plane, mounted on risers above the cab roof, with a pair of can style antennas mounted on the platform. A recent exchange on Trainorders.com included a photo of UP 82 in June 1969 that showed the initial "firecracker" antenna installation on the unit's cab roof. Rob Leachman wrote that he had photographed the unit in RCS service near Bitter Creek, Wyoming, in August 1969 and at that time it was equipped with the later ground plane "table-top" antenna.

Radio control operations were initially tested in 1968 with SD45s in Utah's Weber Canyon, using UP 3622 and 3623. Installation of RCS on additional SD45s began in fall 1968 and continued through 1972, but UP 82 and 82B were the only non-SD45s to receive RCS equipment. The two DD35s received their RCS installation reportedly for use on an ore train between Salt Lake City and Butte, Mont. In his message on Trainorders.com, Rob Leachman wrote, "These were the only Locotrol-equipped double diesels. The intent was to power the seasonal Silver Bow ore train with this set. I have never seen any pictures of that, must have been an incredible show. They removed the RCS equipment from the 82 and 82B within 1-3 years and put them in SD45s, but the table-top antenna platform remained many years afterwards, maybe until the 82 was scrapped." Later in the discussion, he wrote that when he rode UP 82 in 1971, the equipment was operable, but when he next rode the unit, in August 1975, the RCS equipment had been removed. Company roster listings of RCS units during 1975 confirm the 1975 removal date. UP 82 and 82B were retired in December 1979 and January 1980, and both were scrapped by Precision National in late 1980.

Upgraded Electrical Cabinets

Along with the electrical reliability problems other roads were having with their GP35s, UP's own experience with the reliability of the electrical systems in both its GP35s and its DD35s (including the DDA35s) was less than positive. In an attempt to alleviate the on-going reliability problems with the electrical control systems on the DD35 units, in early 1973 two units were selected to be modified with entirely new electrical cabinets purely of UP's own design. Using a combination of components from GE and other manufacturers, UP designed and fabricated new electrical cabinets (front and rear) for DD35 98B. UP 93B received a new cabinet of a slightly different variation only at one end. The new designs were meant to be similar to the fully enclosed and sealed cabinets of the new DDA40X Centennial units and had similar appearance. These new electrical cabinets dictated that the as-built sandboxes for these two units be relocated to the units' sides, again matching the appearance of DDA40X units. These new sandboxes were fabricated by UP's own forces in its Omaha shops. In April 1975, UP DD35 91B was given an improved version of the new electrical system.

Side-Mounted Sandboxes

Although the new electrical cabinet designs were found to make for a more reliable locomotive, the added costs of completely rebuilding the electrical cabinets for the entire DD35 and DDA35 fleet was calculated to be too high. As a stop-gap measure, the railroad decided to refurbish the current electrical cabinets, and to modify the carbodies to alleviate a nagging dust and grit problem that seemed to be the root of most of the problems. As built, the DD35s had their electrical cabinets at the ends of the locomotives; the DD35s at both ends, and the DDA35s at just the rear end. Access to the cabinets was through a side door into a compartment that also contained the units' sandbox. UP had discovered that through normal wear and tear, the welds in the seams of the sandboxes were developing minute cracks and fissures. While there was no structural danger, these cracks were allowing very fine particles of highly abrasive sand to enter the electrical cabinets, causing most of the reliability problems. The fix was to completely remove the sand boxes from the ends, and install new sandboxes mounted to the sides, giving the units an appearance similar to the as-delivered sand boxes on the newer DDA40Xs. The modification was completed by UP's Salt Lake City, Utah, and North Platte and Omaha, Nebraska shops over the period of late 1975 to late 1976, with some variations in completion times due to some units being in storage. While the new sand boxes ended most of the electrical problems, the units were still poor overall performers, and were usually the first to be stored during the late 1970s at times of normal downturns in traffic.

Storage and Retirement

As with most roads, Union Pacific has always been subject to increases and decreases in traffic levels that follow regular business and industrial cycles. As traffic levels fell due to diminished business levels, fewer trains were run, and fewer locomotives were needed. During earlier days, as many as 10 percent of the steam fleet was stored, and during the early years of UP's diesel era, the road also regularly stored unneeded locomotives. The units selected were usually the least reliable or units with the least versatility as to they could be used. In the late 1950s and early 1960s, the Alco FA/FB cab freight units, and Alco PA/PB and Fairbanks-Morse Erie-built passenger units were usually spending more than their share of the time on the dead lines. As these units were retired during the mid 1960s (and traded for new DDA35s), the GP9B units took their place in the storage lines. By the mid 1970s, as more SD40-2s arrived on UP's rails, the road's DD35s were often stored until needed during the regular up-swings in traffic levels.

During mid 1975, of the 170 units in storage, four were DD35s. By June 1977, 24 of the 27 DD35s were in storage, along with three of the 15 DDA35s. The fall increase in traffic brought the units back into service, but the 1978 season was slower than the 1977 season, and the DD35 cab and booster units all returned to the storage lines, where they remained until their later retirement.

Among the first DD35s to be retired was the first one built, UP 72B. It and six other DD35s were retired in June 1979. Nine more units followed in September 1979, including the first DDA35, UP 70. UP 82 and 85B followed in October, and three more DD35s and a single DDA35 were retired in December 1979. A total of eleven units were retired in 1980, between January and May, including seven DD35s and four DDA35s. The last DD35 to be retired was UP 93B. It and the last eight DDA35s were all retired in August 1981, having not turned a wheel in revenue service for two years.

All of the units were sold for scrap. The sales began in October 1979, with the sale of UP 75B to Naporano Iron & Metal in Newark, N.J. Five more units followed in December - sold to Naporano, and to two other companies, Precision National at Mount Vernon, Illinois, and Peaker Services in Michigan. Between May and November 1980, 17 units were sold to both Naporano and Precision, and three were sold to Azcon Hyman Michaels in Alton, Illinois. Seven units were sold in January 1981, again to both Naporano and Precision. In February and April 1982, five units were sold for scrap to Azcon and Naporano, and a single unit was sold to General Metals in Tacoma, Washington. Bargains Galore in Vancouver Washington, bought UP 93B (the last one) and UP 72. Both units were sent for actual scrapping to Simon & Sons in Tacoma, Washington. The last DDA35 to leave UP property was UP 77. It was sold in July 1982 to Erman-Howell, and cut up in their salvage yard in Turner, Kansas. UP 72B, the first DD35 built, remained at Peaker Services facility in Brighton, Michigan, after Peaker had purchased the unit in December 1979. The majority of Peaker's business is to service diesel standby power plants, known as "peaker" units, for the electric stationary power industry, hence the company's name. They were a regular customer of retired EMD units from many roads, stripping the units of parts used in their rebuild of the EMD diesel engines used in stationary power plants. UP 72B was eventually stripped of all its usable components and, according to Trains magazine in mid 1983, the frame was offered by Peaker to the Michigan state highway department for use as a bridge. Although the offer was not accepted, and the unit scrapped in June 1988, it was an interesting possibility that this unique one-piece, 82-foot frame might have served as a highway bridge along some unsung side road in rural Michigan.

The 1981 retirements of the last DD35 and the last eight DDA35s brought an end to an era on UP. The pioneering designs of the 27 DD35 booster and 15 DDA35 cab units that Union Pacific operated between 1964 and 1978 acted as the innovating designs for later, more numerous units. They helped both UP and the builder, EMD, refine the features of double diesel locomotive into a fleet of 47 even more unique DDA40X Centennials that dominated UP's mainlines throughout the 1970s. Unfortunately, no DD35s or DDA35s were saved, but we can remember them for their truly unique features, and the unusual locomotive consists that they were usually part of.

(For additional insight into the delivery of DD35s and Century 855s to UP in 1964, see Jack Wheelihan's comments in "The Inside Track," in the February 1999 issue (Issue #244) of CTC Board magazine.)

Roster Listings

Roster listings of UP's EMD DD35s

Roster listings of UP's GE U50s

Roster listings of UP's Alco C855s