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Locomotive Descriptions and Phases - SD28, SD35, SDP35, SD40X

The EMD SD35 was introduced in 1964 as a replacement for the SD24, making a direct transition from the first-generation carbody introduced with the SD7 to the newer design introduced a few months earlier with the GP35. The 16-cylinder turbocharged 567 engine saw a 100-horsepower increase to 2,500 horsepower, in line with the GE U25C but slightly lower than the ALCO C-628. Compared to the SD24, the cab roof was lowered and the long hood was significantly raised, bringing them to the same height. All the improvements in design introduced between the GP30 and GP35 in 1963 were carried over to the SD35, and the hood was nearly identical to the GP35 save for an additional 3 feet of space between the radiators and engine room doors (making the hood the same length as on the later GP40). The general hood design - 6' wide, 9' 4.25" above the frame, tapered ends, a flat roof, and slightly rounded top corners - was carried through to the SD70 series in the 21st Century.

Although the underframe was the same length as on the SD24 (60' 8" over the coupler pulling faces) the overall design was significantly revised, with a narrow side sill exposing the sand and air piping and a slight "fishbelly" profile above the fuel tank. Unlike later models, the SD35 had the draft gear welded to thick plates below the undeframe that brought it to the same height as on GP units. In 1965, the GP35 adopted the general underframe design and air pipe arrangement of the SD35, but since there wasn't room on the GP35 for the air reservoirs above a flat-topped fuel tank, both models introduced a sloped-top fuel tank profile that would be used on almost all EMD models in the following decades.

The SDP35 was offered as a passenger version, with the rear of the hood extended and the radiators moved forward to make room for a steam generator. A 1,800 horsepower, non-turbocharged (Roots-blown) version, the SD28, was also produced as a replacement for the SD18. It had two exhaust stacks (in place of a single turbocharger stack) and two 48" radiator fans (in place of two 48" fans flanking a 36" fan) but otherwise looked the same as the SD35. Only six were built, reflecting a preference among railroads at the time both for four-axle units and for higher-horsepower engines. Nine test beds for the 645-series engine were also built and were designated SD40X.

While the SD35 did not sell as well as the GP35, it was nonetheless the fastest seller of the SD series up to that point, with combined sales of nearly 400 for the SD35 and SDP35 in only a year and a half of production. However, the 2,500 horsepower rating pushed the limits of a traditional DC generator, and the resulting electrical system, with nine stages of transition (four in series, three in series-parallel and two in parallel) proved complex and unreliable. The 567 engine, while still reliable, was nonetheless nearing its horsepower limit with technology of the time, having nearly doubled from the 1,350 hp of the FT in 1939. In 1966, EMD replaced the SD35 with the SD40, which used the alternator-rectifier electrical system and new 645-series engine first seen in the SD40X test beds. Owing to the complex electrical system and the turbocharger (which was somewhat more maintenance-intensive than the one on later 645 engines) relatively few SD35's remained in service by the year 2000. However, some were updated or rebuilt, such as with the addition of Dash-2 electronics or the replacement of the turbo with a Roots blower.


These phases are based on the SD35 roster compiled by David Thompson. While Thompson's original phases make the transition from Phase 1 to Phase 2 based on the radiator intakes, I have elected to make Phase 2 based on the fuel tank profile, for two reasons: 1) Among the detail changes made to the SD35, the radiator intakes are relatively minor, while the new fuel tank profile involved more significant changes to the underframe, including the brackets suspending the fuel tank; and 2) doing so brings the phases in line with the GP35. For the same reasons, I've listed Thompson's Phase 1c1 and 1c2 as 1b2 and 1b3.

I have grouped Thompson's Phases 2c through 2e all as sub-phases of Phase 2a, since they describe three different arrangements of the handrail stanchions that all appeared within a single NW SD35 order. Thompson's Phase 2d2 (describing a relocated front jacking pad) actually occured just after all the handrail stanchion variations, so I've listed it as Phase 2b1. I've also added a few more details I've observed, such as the blower duct housing and sheet metal extension ahead of the right-side battery box affecting Phase 1c.

See also GP28, GP35 Phases.

Phase 1a 1b1 1b2 1b3 1c 1d1 1d2 2a1 2a2 2a3 2b1 2b2 3
Dates 1964.06 -
1964.09 -
1964.10? 1964.11 -
1964.12 -
1965.02 -
1965.04? -
1965.05 -
1965.08 1965.08 1965.08 1965.08 -
1965.09 -
Central hood doors 1 3 latched 7 latched
Panel under headlight 2 Flat (with headlights) or rounded (with gyrating lights) Rounded regardless of light type
Left side battery box cover 3 2 groups of 3 louvers 2 single louvers
Right side battery box cover 3 groups of 3 louvers, sheet metal extension in front of battery box No sheet metal extension 2 single louvers
Radiator intakes 4 Metal bars Wire screen
Blower duct housing 5 Smooth edges Seam along edges, fewer bolts
Inertial air intake 6 Flat perforated sheet Corrugated
Phase 1a 1b1 1b2 1b3 1c 1d1 1d2 2a1 2a2 2a3 2b1 2b2 3
Dustbin hatch 7 Flat Raised
Fuel tank 8 Flat top Angled top, revised connections to underframe (inconsistent on SDP35)
Handrail stanchions ahead of cab 9 Near battery boxes Closer to stepwells
Handrail stanchions along long hood 9 10, evenly spaced 11, stanchion added at rear, stanchion near cab moved rearward 10, stanchion near cab same place, remainder moved rearward
Front jacking pad 10 Under front face of cab Moved forward (under battery box) Moved rearward (under cab)
Air reservoir piping Exposed Hidden behind air reservoirs
Exhaust stack Narrow Wider (as on 40 series)
Phase 1a 1b1 1b2 1b3 1c 1d1 1d2 2a1 2a2 2a3 2b1 2b2 3

Other details


These two drawings illustrate typical early (Phase 1a) and late (Phase 2b2) SD35's built to similar BO specs. Externally, these represent very generic SD35 variations.

Click on the links to toggle between the images.

Phase 1a -  Phase 2b2

Model variations


Nine locomotives were built as test beds for EMD's new 3,000 horsepower 645-series engine and AR-10 alternator, using SD35 frames. From what I've found, they were not initially given a model designation, but were later classified by EMD as SD40X.

The first, EMDX 434, was built at the start of SD35 production in 1964. The radiator intakes were slightly longer than on the SD35 (but not as long as on the SD40) and there were three closely-spaced 48" radiator fans on the roof. This unit later became GMO 950 and eventually IC 6071, and has been preserved following retirement from active service in 2009.

Six more were built in early 1965 and numbered EMDX 434A-434F, alongside a single GP40X numbered EMDX 433A built on a GP35 frame. As the radiator section of the first test bed was too long for the GP35 hood, 433A and 434A-434F all used canted radiator intakes that were the same length as on the GP/SD35, resulting in three closely-spaced 48" fans located very near the end of the long hood. This radiator design was later used on the GP40P built for CNJ. These units also used the wider exhaust stack housing that would later be used on 40-series units.

The last two (EMDX 434G-434H) were built at the same time as 434E-434F and introduced the radiator design that was ultimately adopted for the SD40, with flat radiator intakes significantly longer than on any of the earlier units, allowing for the fans to be slightly farther apart.

The eight later demonstrators were sold to UP and numbered 3040-3047, in the middle of UP's other SD40 units. One of them (3046) was still in service in 2020 on Wheeling & Lake Erie.

These early SD40 demonstrators are not to be confused with the GP40X and SD40X built in the late 1970s, which were test beds for the EMD 50 series.


Cook, P. (2006). The EMD 567 Engine in the 21st Century. Retrieved January 2020 from

Electro-Motive Division. (1964). Diesel Locomotive Operating Manual for Model SD35 and Model SDP35. Retrieved January 2020 from

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

Strack, D. A History of Union Pacific Dieselization, 1934-1982. Retrieved January 2020 from

Thompson, D. EMD's SD35 and related models - Original Owners. Retrieved January 2020 from