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Locomotive Descriptions and Phases - EMD GP40X

The GP40X was produced from December 1977 to June 1978 as a test bed for several new features and upgrades over the GP40-2, some of which were later incorporated into the GP50. A total of 23 were produced for four different railroads: 10 for ATSF, 3 for Southern, 4 for Southern Pacific and 6 for Union Pacific.

The 16-cylinder 645-series engine used in the GP40-2 was uprated to 3500 horsepower, necessitating larger radiators with canted intakes and a higher speed at full throttle of 950 rpm instead of 900. The overall length (60' 2") and truck bolster centers (35') were both 1 foot longer than on the GP40-2, and the underframe was about 1.5" taller. The GP40X also incorporated EMD's new "Super Series" wheelslip control system, which used Doppler radar to precisely measure locomotive speed and allow controlled amounts of wheel "creep" (slight slipping) to improve adhesion.

While the higher horsepower rating and some of the electrical and structural differences were carried over to the GP50, a number of other details were unique to the GP40X and weren't retained in later models. Both side walkways were raised along their full length, with the left side (as on other EMD units) containing the air duct for the rear traction motors. On the hood, the dynamic brake housing was taller than on other EMD models, pushing down the height of the engine compartment doors. The inertial air intakes were a few inches below the top of the hood roof, which in turn reduced the height of the traction motor blower duct on the hood behind the cab. The radiator fans were an experimental low-profile design; on most units they were replaced relatively quickly by either standard 48" fans (as seen on other EMD models prior to 1980) or later Q-type fans. The hood doors themselves used smaller hinges not seen on other EMD models, and the stairwells had a narrower side sheet than on the GP40-2 or the GP50.

Units built for Southern, SP, and UP remained in service on those roads (or successors) until the early 2000's, after which most were sold to other railroads. Several were significantly modified as rebuilds for CP and SLR. As of 2022, the units built for ATSF remain in service (in rebuilt form) with BNSF.

Truck Designs

GP truck

Units built for ATSF and Southern were built with single-shoe GP (or "Blomberg M") trucks as seen on much of contemporary GP40-2 production. Those on ATSF used the standard "sandwich" of flat rubber pads for the secondary suspension, which provided a rougher ride than the more compliant leaf springs used on the earlier clasp-brake version of the GP truck.

The three Southern units used a different secondary suspension design that also appeared on a portion of EMD F40PH production. In place of the vertical stack of rubber pads, two angled stacks of pads were oriented sideways, largely invisible from the outside but with the rectangular support for the outer pads readily visible inside the swing hanger. The swing hanger itself was the taller version used in the clasp-brake version of the truck. This revised suspension design was a first attempt at improving the vertical ride compliance over the standard single-shoe design. As the remainder of the truck components were the same, this truck did not offer an advantage in weight transfer compared to the normal "Blomberg M" design.

Later, partway through GP50 production, EMD reintroduced leaf springs for the secondary suspension, albeit reduced in height to fit within the shorter swing hanger. ATSF GP40X's were retrofitted with leaf springs, while Southern units received standard "Blomberg M" trucks with flat rubber pads, in line with the Southern GP50 fleet.

HT-B truck

The 10 units built for SP and UP were equipped with an experimental high-adhesion truck, known as the HT-B (for high-traction, 2 powered axles). Compared to the GP truck, the HT-B had taller primary springs (similar to the 6-wheel HT-C truck), a longer wheelbase, and a secondary suspension consisting of angled rubber pads designed to reduce weight transfer between the axles in high-tractive effort situations. The suspension design was similar in principle to the MLW ZWT (zero weight transfer) truck introduced a few years earlier, which may have inspired the design. EMD GP50 specifications list the wheelbase as 9' 9", but other sources also state a wheelbase of 9' 9.5" or 9' 9.75".

The taller primary springs in the HT-B truck required more clearance under the locomotive underframe. While I don't have a diagram listing the frame height for the GP40X, EMD GP50 specifications list a walkway height of 63.5" and 64.75" for versions with the GP and HT-B truck respectively, both with 40" wheels. As the HT-B trucks on the GP40X initially had 42" wheels, that would further increase the walkway height to 65.75", or 2.25" higher than with the GP truck. (In later years, some units received 40" wheels). Comparatively, the GP40-2 walkway height was about 62".

The HT-B and GP trucks were not easily interchangeable. The HT-B used a different center bearing that extended down from the bottom of the underframe, and the traction motor air ducts were shifted to match the longer wheelbase. Owing to the higher ride height, units with HT-B trucks had the height of the couplers and draft gear lowered; photo measurements I've taken of the pilots and draft gear suggest a difference of between 2" and 2.5", which is in line with the specifications for the frame height. The traction motor cable routing was also slightly different between the truck types.

While the taller primary springs increased vertical ride compliance, the lack of swing hangers degraded the lateral ride compared to the GP truck. Unlike with the 6-wheel HT-C truck, which provided a significant improvement in adhesion over the earlier Flexicoil design, the 4-wheel HT-B did not provide a notable benefit over the GP truck in head-to-head testing. As it was a new design, it was more expensive to manufacture - and it eliminated the possibility of reusing trade-in trucks from older units (a significant cost disadvantage). Although the HT-B truck was initially listed as an option for the GP50, it was ultimately not used on any later production.

Railroad Variations

In addition to the variations in truck designs listed above, GP40X production also included features that were typical of the four railroads at the time. Below is a summary of a few of the minor railroad-specific variations; more details are further described below.

Horn Leslie S-3L-R
2x Nathan P5L
at each hood end
Nathan P3
cab-mounted (right side)
Leslie S-3L-R
between radiator fans
(later cab-mounted)
Bell Underframe Long hood end Cab roof (left side) Underframe
Front pilot Standard plow
Large plow
no anticlimber
Large plow
Large plow
narrow sill
no anticlimber
Rear pilot Flat Large plow Flat
MU hose pockets
narrow sill


As on ATSF GP39-2's and SD40-2's, the GP40X's had a cluttered cab roof, with an air conditioner in the center, a Prime 8901 amber beacon on the left side, a Leslie S-3L-R horn on a raised support at the front, and a conduit leading to an antenna plane behind the cab, above the inertial air intakes. ATSF 3800 and 3801 (which apparently arrived a few weeks after 3802-3809) were equipped with Locotrol equipment for use as mid-train helpers, evident externally by the antennas raised on small "boxes" between the second and third radiator fans.

In place of the standard 3600-gallon fuel tank, ATSF units used a 3000-gallon tank, which was about 3' shorter. These were the only GP40X's with standard front plows, so they had standard air piping with the train line shutoff valve in a recess in the pilot face; the other units all hard large plows with modified air piping that moved the shutoff valve up underneath the walkway side frame. The front pilot also featured an anticlimber, and the bell was in the standard location on the left side in front of the fuel tank.

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Like other Southern units at the time, GP40X's 7000-7002 were built with a high short hood and horns at each end. As they were normally operated long hood-forward, the bell was at the end of the long hood. The cab eaves had a drip rail in place of sliding sunshades, and the walkway was illuminated by platform lights on the side of the hood below the exhaust stack and above the radiator intakes. The units had plows at both ends (with correspondingly shorter end handrails) and Type F couplers, along with dual fuel gauges on the fuel tank.

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As-built, the SP units were arguably the most visually distinct. In addition to featuring HT-B trucks, they were built with shrouds over the radiator intakes extending down to the handrails, intended to help draw cooler air to the radiators in tunnels in a similar fashion to the SD40T-2 and SD45T-2 built a few years prior. They had full SP 5-light packages on both ends, consisting of twin headlights, a 2-light clear Gyralite and a single red Gyralite. They also had MU shutoff valves exposed under the walkway side frame rather than behind the steps, a feature adopted on other SP units a few years earlier.

Like the ATSF units, the cab roof had an air conditioner and a conduit to an antenna plane over the inertial air intakes, and they were Locotrol-equipped (therefore having additional antennas between the radiator fans). They also had "L" shaped front cab windows, combining the two normally rectangular windows in front of the engineer. The cab side panel was welded instead of riveted, a feature that would be adopted on Dash-2 models a few months later.

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Compared to the SOU and SP variations, UP GP40X's were relatively "off the shelf" in appearance. Typical of UP units at the time, they had large front plows and small raised fuel gauges next to the fuel fillers. The pilot end platforms were thinner and had a larger, extended MU connector housing underneath. Like SP units, the cab had a welded side panel.

As-built, the horn was on a raised platform between the first and second radiator fans, with an air pipe running on the right side to the rear of the hood. The cab sides were bare except for a standard armrest and a narrow drip rail at the top. Almost immediately, the cab received sunshades, dual mirrors and UP-style wide armrests held by three L-brackets. The horn was relocated to the front of the cab roof, which had become the standard location on UP units around the time the GP40X's were delivered.

There were a couple of minor variations between units. UP 9000 had the vent on the left side of the cab located at the front edge rather than the rear. UP 9002, 9003 and 9005 had 2 additional casting holes near the center of the HT-B truck frames.

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In 2005, CP obtained a series of GP38-2 rebuilds from NRE, four of which (numbered 4521-4524) were rebuilt from GP40X's. The latter three were former UP units, while 4521 was originally SP 7231. The turbocharger was replaced with a Roots blower, reducing horsepower from 3500 to 2000, and the center radiator fan was removed. A paper air filter box appeared in the space formerly occupied by the turbocharger, and two exhaust stacks were added on the hood roof (on either side of the dynamic brake fan). Their carbodies otherwise remained largely the same.

The most significant alteration was the conversion on all four units from HT-B trucks to clasp-brake GP trucks (standard "Blomberg" trucks) which were better suited to switching duties. While NRE was able to make the required changes to the underframe to accept the different bolster design and air duct locations, the draft gear pockets apparently remained unchanged; as a result, due to the lower right height of the GP truck, in photos the couplers are visibly lower than those of the locomotives to which they are connected.


The first GP40X variations I saw in person - and among the first to be significantly modified from their original form - were two former Norfolk Southern (nee SOU) units that arrived on the SLR in 2002. Former NS 7000 and 7002 became SLR 3209 and 3210 respectively (the third Southern unit, 7001, was apparently parted out and sat unused for many years with BDLX or GMTX reporting marks). During rebuilding at NRE, the high short hood was lowered but remained 81" in length (rather than the 88" used on other GP40X production). New cab windows and numberboards were installed that were indistinguishable from factory low-nosed units. Both units wore the bright yellow and black paint scheme used on the SLR before it was purchased by Genessee & Wyoming.

I believe it was at this point that the prime movers were derated to 3000 horsepower; in any case, by the time they arrived on the SLR the engine speed had been reduced to 900 rpm. By this point they also had older-style flat-top fans (in place of either the as-built low-profile fans or later Q-type fans), so they performed - and sounded - very much like a GP40-2.

Less than two years later, the units were rebuilt a second time as "mother" units to power road slug sets, with 3209 and 3210 becoming 3806 and 3805 respectively and classified as "GP40-3". The electrical systems were upgraded and they received extra cable connections on the rear pilot to power the slug units, along with a fuel connection to draw fuel from the slugs (which retained their fuel tanks). In the process they also received the G&W corporate orange, black and yellow paint scheme. Aside from the slug connections and new paint, their outward appearance was relatively little altered, other than the addition of a small rear plow and snow shields over the inertial air intakes.

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Electro-Motive Division. (1984). Model GP50 - 3600 hp General Purpose Locomotive, Specification 8112.

Electro-Motive Division. (1984). SD50 Operator's Manual, 5th Edition. Retrieved December 2022 from

Percy, R. (2008). Southern Pacific GP40X Info Page. Retrieved December 2022 from

Radial Truck Development at EMD [Online discussion]. (2020). Retrieved December 2022 from

Strack, D. (2011). Locomotive Trucks. Retrieved December 2022 from

Strack, D. (2016). Union Pacific's Last Four SD40-2s. Retrieved December 2022 from

Strack, D. (2023). UP Diesel Locomotive Features. Retrieved January 2023 from

Unofficial EMD Homepage. (2005). EMD GP50 Order Numbers. Retrieved April 2010 from