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EMD GP20 History
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EMD GP20 History

A History of General Motors - Electromotive Division
General Purpose GP20 Locomotives

History     Phases     Production Roster     References


The EMD GP20 model was born out of the necessity by EMD to get greater horsepower out of a diesel locomotive. During the late 1950s, trains became heavier, schedules tighter, and there was a call for locomotives with more horsepower. At this point in history, ALCO and General Electric were developing road switchers with as much as 2500 horsepower and for EMD to remain a competitor in the diesel market; they had to develop a locomotive that could match their competitors. Also, as of September 1955, the "ever eager for higher horsepower" Union Pacific, was experimenting with ways to increase locomotive horsepower. Union Pacific turbocharged a pair of GP9s and a GP9B in its East Los Angeles shops for testing with AiResearch turbochargers. This work was done in late 1955 and early 1956. After this experiment proved a success, the Union Pacific turbocharging program was moved to its Omaha Shops. Starting in the Spring of 1959, Union Pacific’s Omaha shops equipped twenty GP9s with turbochargers, ten from AiResearch and ten from Elliot. These units became known as the “Omaha GP20s” (see a great online article written by Don Strack, entitled "Omaha GP20's, Union Pacific's GP9 turbocharging program" for a complete history).

By 1958, EMD had developed a turbocharger of their own, so Union Pacific modified nine GP9s with these turbochargers in the spring of 1959. The development of the turbocharger was a difficult process for EMD because of their use of their two cycle 567 diesel engine. Their solution was to have a two process to drive the turbocharger. For low throttle settings the turbocharger was driven by a planetary-type gear train that rotates at a given ration to the engine crankshaft. Above throttle settings of six or seven, a clutch disengages the gear drive and the turbocharger was driven by exhaust energy. This combination allowed for a rapid engine response to the full range of throttle changes.

Even though EMD was reluctant to incur the possible maintenance headaches of the turbochargers on 4 axle units (because the increase in horsepower was not that great over the current GP9 model) they began production of the GP20 model in November of 1959. They were advertising a savings from roster reductions (3 for 2), a reduced scheduled maintenance time by about 60%, and increased freight speeds with fewer units that would offset the expense of the new turbochargers. EMD was even offering trade in allowances for older units towards the cost of the new GP20s as an incentive for sales. The base cost for the new units was $187,000.

The GP20s are very similar in appearance to the GP7, GP9 and the GP18 in outward appearance. The GP20s can be recognized by the rectangular bulge behind the cab on both sides of the unit for the turbocharger, a single large exhaust stack behind the first cooling fan, an increase in the number of car body louvers, and the use of grid protection on the radiator shutters (like the GP18). The units have rectangular radiator grills and a large 2350 gallon fuel tank (except for the Great Northern units with 900 gallon fuel tanks). This larger fuel tank forced the relocation of the air tanks from the rear of the fuel tanks to a narrow ledge along the top-sides of the fuel tank.

The units were equipped with a 48 inch cooling fan behind the cab, a 48 inch dynamic brake pan top-fan (if equipped with dynamic brakes), a 36 inch cap-top cooling fan and a final 48 inch cooling fan at the rear of the unit. A flared style fan was used on the front and rear 48 inch cooling fans until April 1961, when a pan-top style fan came into use on the cooling fans.

The power plant was a 16 cylinder 567 D2 engine that fed power to four D47 traction motors, each capable of producing 500 horsepower. The stylish and functional low nose (as first introduced with the SD24s and Southern Pacific GP9s) was applied to all units except for those built for the Great Northern and the Western Pacific. The only units without dynamic brakes were the fifteen units produced for the New York Central in 1961. Western Pacific took delivery of the first six production units in November of 1959 (may have not been until December 2, as Virgil Staff states in his book D-Day on the Western Pacific, page 138). The GP20s were produced from November 1959 until April 1962. EMD produced 260 units and all were sold for the United States market.

Not only did the GP20s help launch an era in which the turbocharger was the rule, it also spawned imitations; such as the Milwaukee Road GP20s. These units were rebuilds of former GP9 units equipped with EMD 645 engines to give them 2000 horsepower without the turbochargers. They even went to the extent of placing EMD GP20 model plates on the sides of these units. In all, the success of any locomotive can be measured by its service life. Using this standard, the GP20 has done well, as many of the units worked into their third decade of existence. Many units have been rebuilt and modernized with the latest technologies and are still active on many the of Class II and Class III railroads of the United States.

Unfortunately some units were not so fortunate and found themselves in scrap yards around the country. When the Union Pacific began to retire their GP20s, they decided to save the first production model GP20, the ex Western Pacific 2001. Realizing its history and importance, they donated the unit to the Western Pacific Railroad Museum at Portola, California in April 1985.

EMD GP20 Brochure Cover

Click on the thumbnail to view the EMD GP20 Brochure used for the Introduction of the new model.

GP20 Phases:

Phase Ia

Phase features:

  • Production dates 11/59 thru 4/61
  • 2 flared-top 48" cooling fans
  • 1 cap-top 36" cooling fan (not visible with winterization hatch) ahead of the rear 48" cooling fan
  • 1 pan-top 48" dynamic break fan (if equipped)
  • Side sill perforations for inspections (except the units built for Great Northern)


  • AT&SF 1100 - 1124
  • CB&Q 900 - 935
  • GN 2000 - 2035
  • EMD 5625 - 5628 / SP 7234 - 7237
  • SSW 800 - 809
  • UP 700 - 729
  • WP 2001 - 2010

Phase Ib

Phase features:

  • Production dates 7/61 - 1-62
  • 2 pan-top 48" radiator fans
  • 1 cap-top 36" radiator fan (not visible with winterization hatch) ahead of the rear 48" fan
  • 1 pan-top 48" dynamic break fan (if equipped)
  • Side sill/skirting perforations for inspections


  • AT&SF 1125 - 1174
  • NYC 6100 - 6114
  • SSW 810 -819

Phase II

Phase features:

  • Production dates 1/62 - 4/62
  • 2 pan-top 48" radiator fans
  • 1 cap-top 36" radiator fan (not visible with winterization hatch) ahead of the rear 48" fan
  • 1 pan-top 48" dynamic break fan (if equipped)
  • no side sill/skirting perforations
  • stepped frame sill


  • SP 7200 - 7233

** Phase is an arbitrary designation by rail fans that is used to distinguish the differences in production variations within the locomotive model.

GP20 Production Roster:

Company Quantity Unit Numbers Date
Atchison, Topeka & Santa Fe 75 1100 - 1174 1960  
Chicago, Burlington & Quincy 36 900 - 935 1961  
Great Northern 36 2000 - 2035 1961 Small fuel tanks
New York Central 15 6100 - 6114 1960 No dynamic brakes
St. Louis & Southwestern (Cotton Belt) 20 800 - 819 1960 - 1962  
Southern Pacific 34 7200 - 7233 1960  
Union Pacific 30 700 - 729 1960  
Western Pacific 10 2001 - 2010 1959 - 1960  
EMD Demonstrators 4 5625 - 5628 1959 Became SP 7234 - 7237

** Not included in these totals are 47 UP GP9's that Union Pacific converted to GP20 specifications in its Omaha Shops.

References and other Information:

  • "EMD's GP20" Diesel Era July/Aug. 1998: 38 - 55.
  • Withers, Paul K. GP20 and SD24 - EMD's Turbocharged Duo, Halifax.: Withers Publishing, 1998.
  • Soloman, Brian EMD Early Road Switchers: GP7 - GP20 Locomotives, Minnesota.: Traintech, 2006.
  • Kamm, Al Jr "Electro-Motive's GP20" Model Railroader July 1963: 36 - 38.
  • Strack, Don "Union Pacific Turbocharged GP9s" Diesel Era November/December 1996 and The Streamliner July 1988 "Omaha GP20's, Union Pacific's GP9 turbocharging program".
EMD GP20 Engineer Area View  EMD GP20 Second Control Stand Area View

Here are two links to photographs of the interior views of a GP20.

EMD GP20 Operating Manual Cover

A recent addition to my collection is a copy of a GP20 Operating Manual. As stated in the introduction, this manual was prepared to serve as a guide to railroad personnel engaged in the operation of the 2000 horsepower General Motors Model GP20 turbocharged diesel-electric locomotive.

There are a number of pictures of Remaining EMD GP20s that are still active today on many of the U.S. class II and class III railroads, along with new versions of GP20s or GP20 rebuilds. They can be found on the Railpictures web site, go to and select GP20 for the model you wish to view.

Drawing WP GP20
Drawing UP GP20
Drawing GN GP20
Drawing BN GP20
Drawing CBQ GP20
Drawing EMD GP20
Drawing SP GP20
Drawing ATSF GP20
Drawing NYC GP20

Images courtesy of Jacques Bélander©2000