Union Pacific Switchers and Slugs

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This page was last updated on June 26, 2013.

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Although Union Pacific Railroad was an early user of internal-combustion motive power, the company did not make a total commitment to the Diesel-electric locomotive until after World War II. During the early days of Dieselization, the road followed the path of many other railroads of the era. It purchased small orders of units to power its passenger trains - a streamlined, articulated passenger train set, the M-10000, in 1934; additional articulated train sets in 1935 and 1936 (M-10001 through M-10006); EMC E2s in 1937; and E3s in 1939 and 1940.

In late October 1939, UP tested a pair of Electro-Motive Corp. Diesel switch engines, a 1,000-horsepower NW2 and a 600-horsepower SW1. Although the test resulted in a 10-unit NW2 order in 1940, the road was still firmly committed to steam power, including modern, dual service 4-6-6-4 Challengers which began arriving in 1936, and the 4-8-8-4 Big Boy which entered revenue service in 1941. Union Pacific entered World War II with only a handful of NW2 Diesel yard switchers and its fleet of streamlined, Diesel-powered passenger trains, together with a relatively modern fleet of steamers to handle a majority of the freight and passenger duties. Many letters went back and forth in 1940 and 1941 between the president of the railroad, William M. Jeffers, at his headquarters in Omaha and the corporate offices in New York, discussing possible motive power cost reductions, without the high cost of mainline Dieselization akin to AT&SF's wholesale embrace of EMC's Model FT, 5,400-horsepower road locomotive. Jeffers was trying to convince the board of directors that although the tried and true designs of modern steam locomotives were the best place to spend money, railroad traffic would be increasing due to the coming war in Europe, and Union Pacific needed to expand its motive power fleet. The purchase of, at the least, Diesel yard switching power would be a step in the right direction, without losing the cost advantage of mainline operations that used coal-fired, steam power, burning coal from railroad-owned Rock Springs, Wyoming, coal mines. Steam powered yard operations didn't use near the quantities of coal as the mainline operations. A fluctuating level of leadership and a resistance to change and innovation in both Omaha and New York kept Union Pacific away from mainline Dieselization in the early 1940s. But the reduced maintenance of Diesel operations over steam operations, along with the Diesel's other advantage of fewer locomotives doing the same work, got New York's attention, albeit for switching power only. This, with the increasing pressure from large cities to reduce smoke emissions, along with growing Dieselization of competing railroads, was the catalyst in UP's receiving increasing numbers of non-passenger Diesel power. By war's end, UP owned 35 NW2s, 19 ALCo S-2s, and five Baldwin VO-1000s, with 35 additional S-2s already in delivery.

After the success of its 96-unit NW2 fleet, Union Pacific continued on the path to total Dieselization of its yard switching operations with the purchase of 25 EMD SW7s in 1950. This latest Electro-Motive Division offering joined the fleet of NW2s, ALCo S-2s, Baldwin VO-1000s and DS-4-4-1000s, and Fairbanks-Morse H10-44s that were already switching freight with efficiency unknown just a decade earlier. The purchase of TR5 and Baldwin DRS-6-4-1500 and AS-616 heavy switchers in 1948 through 1952, along with the arrival of SW9s and ALCo S-4s in the mid-1950s sealed the fate of steam-powered switcher operations as UP joined the rest of America's railroads in purging its rails of steamers.

Union Pacific's ownership of 179 early EMC/EMD NW2, SW7, SW9, and TR5A and TR5B switchers contributed greatly to the total Dieselization of the railroad by early 1959. Parallel with the purchase of its early Electro-Motive fleet, the railroad also purchased 54 ALCo S-2s, 45 S-4s, six Baldwin VO-1000s, five Baldwin DS-4-4-1000s, five Fairbanks-Morse H10-44s, and a single GE 44-tonner, along with a single Baldwin DRS-6-4-1500 and six AS-616 heavy switchers. These additional 123 non-EMC/EMD switching locomotives, making for a peak Diesel switching fleet of 302 units, allowed UP to completely retire its steam switcher fleet over the decade of 1947-1957.

The continuing need for a fleet of purpose-built yard switchers in the late 1970s led Union Pacific to its unique SW10 design, rebuilt from SW7s, SW9s and TR5As. UP's switcher fleet began shrinking with the changes of operations brought on by less governmental controls, along with a consolidation of operations of 1982-merger partners Western Pacific and Missouri Pacific. The new operations demanded that switchers be more powerful and versatile. Missouri Pacific came to the merger with a large fleet of modern 1,500-horsepower MP15DC units, and the increased power of the ex-MoPac MP15s was appreciated, as was the added flexibility in operations. Through the merger in 1982, the railroad had assembled a healthy fleet of modern Electro-Motive Division SW1500 and MP15 switchers, without purchasing a single unit. In 1985, more MP15s were purchased second hand from eastern road, Pittsburgh & Lake Erie. Still more MP15s came to UP during its merger in 1988 with Missouri-Kansas-Texas (four MP15ACs), and with Chicago & North Western in 1995 (14 MP15DCs). Thirty-two second hand MP15ACs were purchased in 1992, making for a 197-unit fleet in 1996 of 61 SW10s, seven ex-WP and ex-MKT SW1500s, and 129 ex-MP, ex-MKT, ex-MILW, and ex-C&NW MP15s.

With 1996 comes a merger with Southern Pacific, and its fleet of 12 MP15DCs and 57 MP15ACs, along with a decreasing number of SW1500s. During 1996, UP's combined fleet of MP15 models will increase from 129 to 198, representing almost half of all MP15 production of 463 units, and together with the remaining numbers of SW10s and SW1500s, is by far the largest switcher fleet in the nation.

EMD NW2s, SW7s, SW9s, and TR5s  -- The stories of EMD switchers on UP, from the first NW2 in 1939 to the last SW9 in 1953, including the TR5 Cow and Calf sets of 1951.

EMD/UP SW10s -- The story of UP's unique SW10 rebuilt switchers, updated through 1999.

EMD SW1500s, MP15DCs, and MP15ACs -- The stories of the former WP, MP, and MKT SW1500s, the former MP MP15DCs, and the former EMD, MKT, and Soo Line MP15AC units on UP.

Alco, Baldwin, Fairbanks-Morse, and GE Switchers -- The stories of Alco, Baldwin, Fairbanks-Morse, and GE switchers on UP.

Roadway Switchers -- UP's tie plant locomotives.

Yard Slugs -- The stories of the eight UP yard slugs, and the 23 former MP yard slugs on UP.

Road Slugs  -- The story of UP's use of road slugs.

Articles and Book

Each of these articles is an updated and expanded version of text first completed in August 1994 to December 1995, and used in a series of articles in Diesel Era magazine:

"Union Pacific Early EMD Switchers - Part 1, The NW2" Diesel Era, Volume 6, Number 1 (January/February 1995)

"Union Pacific Early EMD Switchers - Part 2, SW7s and SW9s" Diesel Era, Volume 6, Number 2 (March/April 1995)

"Union Pacific Switchers - Part 3, The SW10s" Diesel Era, Volume 6, Number 2 (May/June 1995)

"Union Pacific Late EMD Switchers" Diesel Era, Volume 6, Number 4 (September/October 1995)

The original text of these four articles was used as part of the book "Union Pacific Switchers and Slugs" by Don Strack. The book has 116 pages; 238 black and white photos; 37 color photos; and full rosters of all of UP's EMD, Alco, Baldwin, and GE switchers, plus its yard slugs and road slugs. (Withers Publishing Co., 1996; ISBN 1-881411-11-7)


The original book project would not have been possible without the assistance of the Union Pacific Railroad and its employees, especially Don Snoddy and Larry Evans, along with the untiring support of certain employees who wish to remain anonymous. You have my heartfelt thanks. Other much appreciated help came from Warren Johnson, Ryan Ballard, Mark Hemphill, and Warren Calloway. Thanks to all of you. Thanks also to Paul Withers for his editing and page layout efforts.

The project began in early 1994 when Paul Withers asked if I could furnish him with about a thousand words to go with some UP switcher photos for a small article in Diesel Era. That small article grew into a four-part series, and along with other added material and photos, became the book that was published in 1996.