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Locomotive Phases - EMD GP38, GP39, GP40

The GP40 was introduced in 1966 as an evolution of the GP35. Along with the GP38 and similar SD-series units, it marked the introduction of EMD's 645-series engine, which used the same engine block dimensions as the 567 series but incorporated modified power assemblies with a larger cylinder bore. In the GP40, a 16-cylinder turbocharged version of the engine produced 3,000 horsepower. The GP40 also used an alternator-rectifier electrical system, addressing one of the biggest reliability concerns of the GP35, in which the DC generator required sixteen stages of transition to handle a 2,500 horsepower output. The GP38 was a 16-cylinder, non-turbocharged (Roots-blown) model that produced 2,000 horsepower, initially using a DC generator and later (in the GP38AC) using the same AR10 alternator as the GP40. The GP39, introduced a couple of years later, used a turbocharged 12-cylinder engine producing 2,300 horsepower.

Both the GP38 and GP40 were fairly strong sellers, together accounting for more than 2,000 locomotives built. Only 23 GP39's were built, most for Chesapeake & Ohio. In 1972, EMD introduced an updated "Dash-2" series, replacing the GP38, GP39 and GP40 with the GP38-2, GP39-2 and GP40-2 respectively.

Many GP38's and GP38AC's remained in service with their original owners (or successors) well into the 21st Century. GP40's in their original form started to disappear from Class-1 railroads in the 1990's, but many were rebuilt or continued in service on smaller railroads. A fairly large number of GP38's and GP40's have received upgraded Dash-2 (modular) or Dash-3 (microprocessor) electrical systems, and a number of GP40's have been converted to GP38 variations by the replacement of the turbocharger with a Roots blower.

Transition: From Phase 2 GP35 to GP38/GP40

The increase from 2,500 to 3,000 horsepower necessitated a larger radiator section with three 48" fans in place of the one 36" and two 48" fans on the GP35. The GP40 adopted general hood dimensions and radiator design of the last two SD40X demonstrators, which were based on the SD35; this increased the overall length from 56' 2" to 59' 2". The rear truck was moved 1' inward from the rear pilot face, making it symmetrical with the front truck and resulting in 34' truck centers (compared to 32' on the GP35). The longer frame allowed for a longer fuel tank, which increased in capacity to 3,600 gallons, although smaller sizes were also used. The walkway side frame was 5" tall above the air reservoirs, compared to 3" on the late GP35 (or 7" on the early GP35) and air piping was relocated inboard of the air reservoirs (rather than on the outside).

On the hood, the dynamic brake fan and intakes were moved rearward, and the taper of the dynamic brake hatch was made steeper at the rear than the front. Aside from this and the longer radiator section, the cab and hood were otherwise the same between the late GP35 and early GP40. As on the GP28 (the non-turbocharged version of the GP35), the GP38 used the same hood design as the GP40, but incorporated shorter radiator intakes with only two 48" fans and two exhaust stacks in place of a single turbocharger stack.


These phases are based directly on the GP38, GP39 and GP40 rosters compiled by David Thompson, with additional details from my own observations.

Numbers correspond to the illustrations shown below.

Phase 1a1 1a2 1a3 1a4 1b 2a1 2a2 2b 2c 3
Dates 1965-11 -
1966-03 1966-04 -
1967-06 -
1967-07 -
1967-09 -
1967-09 -
1968-06 -
1971-04 -
Cab sub-base doors 1 Smooth Ribs on conductor's side Ribs on both sides Ribs made full-width (rather than ending before door edges)
Fans 2 Flat center cap Open-top
MU receptacle housing 3 Short, wide, angled side Tall, narrow, angled bottom (appeared on some late Phase 1 units)
End plates Small tabs at bottom steps, end plates do not extend below steps Angled extensions at bottom steps, end plate extends to bottom of pilot No extensions at bottom steps (as on Dash-2 units)
Footboards 4 Flat plate on side extending behind pilot end plate (as on GP35) Beveled plate on side flush with pilot end plate (as on Dash-2 units)
Steps 5 High-clearance on most units (approx. 17.5" above rail); short stairwell side sheet with extension on lower-clearance steps Lower-clearance on many units (approx. 12" above rail); stairwell side sheet height varies with step clearance
Phase 1a1 1a2 1a3 1a4 1b 2a1 2a2 2b 2c 3
Fuel tank supports 6 Triangular brace Pentagonal brace (rectangle with angled corner) additional plates bolted to underframe (absent with smaller fuel tanks)
ECAFBElectrical cabinet air filter box, located on the left behind the cab
No (unless subsequently added) Yes
Short hood grabirons 8 Top-mounted, straight Bottom-mounted, U-shaped (appeared prior to 1968-02 on LN units)
Radiator intake subscreen 9 Diamond mesh Square wire grid
Blower duct 10 Flat Two horizontal ribs
Front taper on dynamic brake housing 11 Short, angled Longer, nearly straight
Handrail stanchions 12 Uneven spacing (closer at rear), bottoms not lined up Even 48" spacing, bottoms lined up
Phase 1a1 1a2 1a3 1a4 1b 2a1 2a2 2b 2c 3

Model variations

Other details


These two drawings illustrate typical early (Phase 1a3) and late (Phase 2c) GP40's built to similar BO/CO specs. Although these units are fairly generic, the Phase 2c version has a few railroad-specific details, such as rock deflector pilots (in place of the later-style footboards) and a high-mounted bell.

GP40, Phase 1a3

GP40 Phase 1a3

GP40, Phase 2c

GP40 Phase 2c


Foster, Gerald. (1996). A Field Guide to Trains. New York, NY: Houghton Mifflin.

Sarberenyi, R. EMD's GP38 and GP38AC - Original Owners. Retrieved March 2008 from

Thompson, D. EMD's GP40 - Original Owners. Retrieved March 2008 from

Unofficial EMD Homepage. (2006). EMD GP38, GP38AC Order Numbers. Retrieved December 2018 from

Unofficial EMD Homepage. (2004). EMD GP40 Order Numbers. Retrieved December 2018 from