EMD Traction Motors

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This page was last updated on June 9, 2026.

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EMD Traction Motors

Here is a timeline of the locomotives that introduced each EMD D-series traction motor model, from the pioneering D7 to the high-horsepower D87. The timeline includes traction motors that could power heavy road-freight locomotives, high-speed passenger units, and humble yard switchers.

Some observers have wondered why the EMC/EMD traction motors were so small, compared to the other early traction motors of other builders such as General Electric and Westinghouse. Jack Wheelihan wrote, "Going back into the 'Pioneer Zephyr' era, the wheels were 36-inch diameter, thus requiring a traction motor size that would clear rail-heads and street crossings. It wasn't until 1938/1939 with the advent of the FT that 40-inch diameter wheels became the standard for all freight diesel units, but the overall size of the EMC/EMD traction motor was based upon use with 36-inch diameter wheels."

Early Years: EMC Used GE and Westinghouse

Before 1938, Electro-Motive Corporation (EMC) was largely a design and marketing firm, not an electrical manufacturer. Early on, they didn't make their own traction motors. Instead, early EMC models, like the Winton-engined switchers, relied on electrical gear sourced directly from their future competitors, General Electric and Westinghouse. The early EMC motor cars used GE 287E or Westinghouse traction motors.

Standard Gauge: The Heavyweight Battle

Once EMD began manufacturing its own electrical equipment (starting with the D7 motor and evolving into the legendary D77 and D87 series), their standard-gauge motors were roughly comparable to the competition, although a bit smaller in size. GE generally held the edge in raw mass of copper components and continuous electrical capacity. And the Westinghose motor was also heavier than the EMD D-series motors.

The GE 752

Although GE's 752 traction motor was introduced in 1946, EMD enjoyed the benefit of first-mover status, with continued production beginning in 1938 and lasting throughout the war years.

The GE 752 motor was heavily utilized by Alco before GE built its own road locomotives. The GE 752 was widely considered by many in the industry as the gold standard for heavy DC traction. It was a massive, overbuilt motor weighing roughly 7,000 pounds bare. Union Pacific took advantage of the cost savings of trade-in GE 752 motors in their trade-in Alco locomotives, and had EMD use the GE motors in fifteen DDA35 locomotives built in 1965.

While EMD's D-Series was packaged to fit standard locomotive trucks, EMD's DC motors historically had a slightly lower electrical capacity compared to the rugged GE 752. For example, during the 1960s and 1970s, an EMD D77 was rated for about 950 amps continuous, while the GE 752 could handle 1050 amps. This internal capacity difference is part of what gave GE units their reputation for superior low-speed lugging and higher amperage limits.

The GE 752 proved to be an incredibly robust, high-capacity, and reliable motor. When General Electric eventually split from Alco in 1953 to build its own mainline road locomotives (starting with the "U-Boats" in the 1960s), the GE 752 remained their standard traction motor. The core design was so successful that modified and improved variants of the 752 (such as the 752AF, 752AH, etc.) were manufactured for domestic North American locomotives well into the 2000s, and they are still produced today for specific industrial and international applications.

The Westinghouse 370

Westinghouse's 370 traction motor was a massive motor meant for heavy road service. The 370 weighed in at around 6,620 pounds bare (and about 7,400 pounds with the gear and case). Much like the GE 752, the Westinghouse 370 was developed to meet the demands of the post-WWII motive power market. As locomotive builders moved past the 1,000-horsepower barrier into heavier, higher-horsepower road and transfer locomotives, they needed a more robust traction motor capable of handling higher continuous tractive effort without overheating.

The initial production model in 1942, the 370-A, was followed in 1945 by the 370-B, then the 370-F in 1946, and again by the 370-G in 1950 (with new axle cap lubricators), the 370-DZ in late 1950 (which introduced advanced silicon insulation), and the 370-GL in 1951 (modified for three-axle trucks). The 370-D, 370-DL, and 370-DE followed in 1951–1953, with larger axle caps for larger diameter axles.

Westinghouse supplied electrical gear to several major builders competing with EMD and Alco/GE. The 370 series became the standard heavy-duty motor for builders like Baldwin, Lima-Hamilton, and Fairbanks-Morse.

Westinghouse unexpectedly announced it was exiting the heavy traction equipment market in 1953/1954. This forced the remaining builders who relied on the 370, primarily the newly formed Baldwin-Lima-Hamilton (BLH) and Fairbanks-Morse, to scramble and redesign their locomotive frames to accept General Electric electrical gear instead.

EMD won the race due to its standardization, from the earliest designs of the late 1930s through to the later designs of the 1980s, with incremental improvements along the way that kept their customers coming back with large orders.

Narrow Gauge: Less Of A Battle

In the heavy North American standard-gauge market, EMD's DC motors were slightly "smaller" in terms of continuous electrical capacity compared to the massive GE 752. However, in the realm of narrow-gauge railroading, GE and Westinghouse were the ones capable of producing physically smaller, more compact axle-hung motors decades before EMD achieved the same engineering feat.

In the export and narrow-gauge market, EMD's traction motors were actually larger (or at least much less adaptable) than those of GE and Westinghouse. By the mid-1940s, both GE and Westinghouse had successfully engineered axle-hung traction motors small enough to fit directly onto 36-inch narrow-gauge trucks. EMD's standard export motors (like the D29) were too bulky to fit anything narrower than meter-gauge (39.37 inches).

To compete in the 36-inch gauge market, EMD had to get creative. For models like the GA8, rather than building a physically smaller motor, EMD mounted standard traction motors lengthwise up inside the locomotive frame and used automotive-style drive shafts to power the axles. EMD did not engineer a true 36-inch gauge axle-hung traction motor (the D36) until they built units for Colombia in the early 1960s.

Standardization

One of EMD’s greatest structural successes was ensuring backwards compatibility. The external physical frame geometry of the early models remained remarkably consistent. This meant railroads could smoothly upgrade an old F-unit or early GP road switcher during major overhauls by sliding a newer D47 or D77 motor into the original trucks.

EMD's Catalog 90, its master parts catalog covering the 1950 to 1965 period, showed that the D57 traction motor as a one-for-one "unit exchange" direct replacement for the earlier D7 through D47 motors. "Model D57 traction motors are no longer available and are replaced by current production model D67 traction motor. D57 traction motors are available only on a unit exchanged basis."

EMD's Catalog 190, its master parts catalog covering the period after 1965, showed that the D77 traction motor was the direct replacement for the D57 and D67 motors, "Model D57 and D67 Traction Motors are no longer available and are replaced by current production model D77. D57 and D67 Traction Motors are available only on an Unit Exchange basis."

Locomotive Production and Traction Motor Introduction Timeline

Motor Model Primary Road Locomotive(s)
(Year Introduced)		 Primary End-Cab Switcher(s)
(Year Introduced)
D7 EMC EA/E1, E2, E3, E4, E5, E6, E7 (1938-1945) EMC NW, EMC SW, EMC NC, EMC NC1, EMD NW2, SW1 (1939-1949)
D17 EMD F2, F3 (1945-1946) EMD SW7 (1949-1950)
D27 EMD F7, GP7, BL2 (1949) EMD SW9, SW600, SW900, SW1200 (1950-1966)
D37 EMD GP9, SD9 (1954-1956) EMD SW1200, SW900, SW8 (late production), GMD SW900 (after 1954)
D47 EMD SD18, GP20 (1959-1960) None
D57 EMD GP20 (late), SD24 (1960-1963) None
D67 EMD GP35 (1963-1966) None
D77 EMD SD40-2, GP40-2, SD38-2 (1972) EMD SW1500, MP15DC, MP15AC (1966-1984)
D77X EMD DDA40X "Centennial" (1969) None
D78 EMD GP38-2, GP39-2, SD38-2 (1972) EMD SW1500, MP15DC, MP15AC (1966-1984)
D87 EMD GP50, SD50, SD60 (1980-1985) None
D87B EMD GP60, SD60 (1984)

None
D90 EMD SD70 (1992) None
D100 (Rebuild only)(late 1990s) None

Key Models and Applications

D7 (1938-1945)

The bedrock of the EMD freight revolution, debuting in the FT cab units. Rated at 700 amps, it was tightly constrained by early varnish-based insulation. The 1,350 hp of an FT divided among 4 axles meant each D7 absorbed 337.5 horsepower.

A pioneering motor for the EMC E series of passenger locomotives and early switchers like the Model 40, it set a physical size standard that future D-series motors could fit within.

The pioneering D7 motor powered a wide variety of EMD's earliest switchers. Specifically, the D7 was fitted in NW series (like the 1,000-hp NW5), early SW series, as well as the NC and NC1, and later SW1 and NW2 models.

D17 (1945-1946)

Debuted simultaneously with the transitional F2 and the significantly revised F3, marking a key post-FT evolution for EMD.

Introduced silicone insulation advances.

This motor was a direct fit in the SW7, which entered production as a 1,200-hp replacement for the 1,000 hp NW2.

D27 (1949)

The F7 cab unit and the GP7 road-switcher both used the D27 motor at their launch, showing EMD's strategy of parts commonality across different designs.

F3s with D27 traction motors were known as, and shown in later EMD documents as F5s.

The versatile D27 was widely used across many first-generation switchers. This includes the SW9, SW600, SW900, and SW1200. Notably, the MRS-1 military road switcher also used six D27 motors.

D37 (1954)

Its introduction with the GP9 marked EMD’s upgrade from the "567B" to the "567C" engine series.

This motor was introduced with the 1,750-hp GP9 in 1954 and quickly became the standard for upgraded switcher models like the SW900 and SW1200, as well as other light switchers like the SW8.

Rolled out alongside the 1,750 hp GP9 and SD9 lines. In a GP9, each motor took on 437.5 horsepower, while in a 6-axle SD9, the load dropped to 291 horsepower per axle, significantly reducing thermal strain on heavy mountain drags.

D47 (1959)

Engineered to handle the heat of EMD’s first turbocharged locomotives, such as the 2,000 hp GP20 and 2,400 hp SD24. It crossed the 900-amp continuous boundary, translating to a beefy 500 horsepower per axle under a 4-axle GP20 configuration.

D57 (1962)

This model featured completely redesigned glass-seamless insulation to handle the massive current spikes generated by the 2,250 hp GP30 and 2,500 hp GP35. In a GP35, 4 motors had to convert 625 horsepower each, testing the absolute thermal limit of the older frame footprint.

D67 (1966)

The final evolutionary step before the introduction of the standard Dash 2-era D77. Reaching 950 amps continuous, it stabilized the fleet during the launch of the early 3,000 hp SD40 and GP40 series. Under a 4-axle GP40, it endured a punishing 750 horsepower per axle, which directly necessitated the massive cooling and ventilation redesign that ultimately birthed the D77 frame.

D77 (1972)

The definitive motor for EMD's revolutionary "Dash-2" line, starting with the SD40-2. It offered over twice the power of the D7 while maintaining the same physical envelope.

The SW1500 and the MP15 series were the final generation of end-cab switchers. They typically used D77 or D78 motors, often designated as D77/78, reflecting the commonality between the two. They used a 62:15 gear ratio for a balance of starting tractive effort and a top speed around 60-65 mph. The SW1500 produced 62,000 lbs of starting tractive effort.

D77X (1969)

A unique, experimental high-capacity variant developed exclusively for the massive DDA40X "Centennial" locomotive, designed to harness its immense 6,600 total horsepower.

D78 (1980)

A direct, performance-upgraded replacement for the D77. The D78 used the D87 armature, with its additional copper windings which in-turn allowed cooler running in the D77 frame.

The EMD D78 traction motor was first made available in 1980, arriving as a specialized transitional component right at the turn of the decade. Its introduction was directly tied to a specific engineering challenge, with EMD finalizing its next-generation D87 traction motor for the upcoming 50-series locomotives (like the GP50 and SD50).

The upcoming D87 featured advanced, lower-resistance copper armature windings that ran significantly cooler under heavy loads. However, the D87 also featured a redesigned external frame and different mounting clearances, which meant it could not be dropped into older Dash 2 or pre-Dash 2 locomotive trucks without extensive, costly modifications.

To offer railroads a high-performance upgrade path for their massive, existing fleets of SD40-2s, GP40-2s, and older units, EMD created the D78 as a hybrid solution. They took the newly designed D87 armature and engineered it to fit perfectly inside the standard, ubiquitous D77 stator frame housing.

Because it was born out of this transition, the D78 became available in two ways starting in 1980. First as a remanufacturing / Unit-Exchange option: Railroads sending failing or worn-out D77 motors to EMD's La Grange or regional rebuild facilities could specify a D78 upgrade. EMD would rewind the old D77 frame with the cooler-running D87 rotor specifications. Later production runs of Dash 2 models and specific export/custom orders built in 1980 and beyond could be specified with factory-fresh D78 motors.

D87 (1980)

Developed for the higher 3,500-3,600 hp demands of the GP50, delivering increased starting and continuous tractive effort to handle the extra power. Its D87 motors provided a significant power boost, enabling a starting tractive effort of 65,000 lbs.

D87B/D87BTR (1984)

The D87B was introduced alongside the debut of EMD's 60-series demonstrator fleet (the SD60 and GP60) in 1984, with full commercial production hitting the rails in 1985.

The D87B refined the electrical and physical design of the base D87. The D87BTR designation marked a critical mechanical milestone, known as the Bearing Tapered Roller. This variation eliminated the old friction support bearings and oil-wick assemblies that rode directly on the axle, replacing them with a low-maintenance, grease-lubricated tapered roller bearing housing (often referred to as a "U-tube" design). It became standard equipment on the GP60 fleet and late-production SD60 orders in the late 1980s.

D90/D90TR (1992)

The D90 and its roller-bearing sister, the D90TR, made their debut in late 1992 with the launch of the landmark SD70 and SD70M locomotives. Unlike the D87 lines, the D90 introduced a physical change in geometry by increasing the physical centerline distance between the armature and the axle. This geometry tweak allowed EMD to squeeze a larger, more robust bull gear onto the axle—giving the 4,000-horsepower SD70 line a massive boost in low-speed, continuous tractive effort without stripping or overheating the gear teeth.

The D90 and D90TR were introduced to provide a heavy-duty DC option as engine horsepowers pushed toward 4,000 hp (such as late-production SD60s and early SD70 DC units). The D90 featured enhanced, proprietary insulating materials for superior ground protection, completely redesigned stator coils, and vented coil supports that significantly elevated cooling performance compared to the 87-series. Like its predecessor, it was available with the tapered roller bearing axle support (D90TR).

D100 (late 1990s) (Rebuild Standard)

The D100 never debuted as original factory equipment on a brand-new locomotive model line. Instead, it was introduced by EMD/Progress Rail in the late 1990s and early 2000s as the absolute pinnacle and final evolution of EMD's factory DC traction motor line, and as the ultimate aftermarket catalog upgrade and factory remanufacturing specification.

As major Class 1 railroads began managing massive aging fleets of DC-traction SD70M, SD60, and overhauled Dash 2 locomotives, EMD introduced the D100 to push DC thermal limits to the absolute ceiling. It incorporated standard vented coil supports, advanced proprietary insulation compounds for superior ground-fault protection, and more robust arc-resistant bands. It was designed to slide into the standard modern DC "box size" but handle the extreme power of modern heavy freight duty cycles.

The D100 integrated all of the thermal, insulation, and mechanical bearing advancements engineered during the late 1980s and early 1990s. It was designed to maximize reliability and push continuous current limits as high as functionally possible within the standard DC motor footprint, serving as the heavy-haul DC baseline right up until AC traction became the standard for Class 1 railroads.

(EMD's first AC locomotives were the SD60MAC demonstrators of 1991–1992, with the first production units, the SD70MAC, coming in 1993.)

D7 through D67 Compared

The earlier D-series models (D7 through D67) were the foundational workhorses that progressively increased in power and sophistication. The table below organizes the available data and highlights the key engineering trends.

The major differences as the motors evolved, was more copper, better insulation between copper components, and better ventilation.

Table comparing EMD D-Series Traction Motors (D7 to D67).

(Research is not complete; comment and correction welcome.)

Feature D7 D17 (D17-B) D27 (D27-B) D37 D47 D57 D67 / D67B
Power Rating 337–375 hp 375-500 hp 375-500 hp 437-583 hp 450-600 hp 562-750 hp 500-750 hp
Years Used / Key Era 1938–1946 1945–1949 1949–1954 1954–1959 1958–1963 1960–1963 1963–1966
Current Ratings
(continuous) 		approx. 700 amps approx. 800 amps approx. 825 amps approx. 835 amps approx. 900 amps approx. 925 amps approx. 950 amps
Primary Applications E-units EA, E1–E7 F2, F3, BL2 F7, GP7, BL2 GP9 SD18, GP20, GP9B, etc. GP20 (late), GP30 GP35

Overarching Similarities: The Backbone of EMD's Philosophy

The progression and evolution from the D7 to the D67 tells a clear story of refinement to handle ever-increasing locomotive horsepower.

The Early Foundation (D7–D27): This generation is less about raw power and more about establishing a robust, reliable standard. The D7 (349 HP) was a pioneering design, and its basic dimensions were reportedly a copy of a pre-war GE motor. Crucially, these core dimensions remained largely consistent through to the D77, meaning a D77 could physically fit in the space of a much older locomotive.

The "Family" of Motors (D37–D67): The post-war models D37 through D67 form a distinct family. They are so closely related that they share many critical internal components, including:

While power increased, EMD's genius lay in evolutionary design and parts commonality.

D77, D77X, D87 Compared

The D77, D77X, and D87 traction motors represent the progression of EMD's direct current (DC) traction motor technology, designed for higher power and reliability in the transition to high-horsepower locomotives. The key difference is that the D87 was a significant upgrade for next-generation locomotives like the SD50, with the experimental D77X tested in the massive DDA40X locomotive.

Table comparing EMD D-Series Traction Motors (D77, D77X, D87)

(Research is not complete; comment and correction welcome.)

Specification EMD D77 EMD D77X EMD D87
Type / Design Series-wound DC motor with a solid iron frame. A higher-capacity variant of the D77. DC motor with higher current rating and construction improvements.
Rated Power Approx. 700 hp (522 kW) Approx. 825 hp (615 kW) Approx. 830 hp (620 kW)
Rated / Continuous Current 1,050 A 1,050 A 1,170 A
Common Gear Ratios 58:19, 62:15, 57:20 59:18 62:15, 70:17, 56:21
Primary Applications EMD "40-Series" & early "50-Series" (GP40, SD40, SD45-2, SD38-2, etc.) EMD DDA40X "Centennial" locomotive (used 8 per unit) EMD "50-Series" & later (GP50, SD50, SD60, F59PHI, etc.)

Key Differences and Similarities

Beyond the numbers, there are critical engineering details that set these motors apart.

Evolution of Power and Current Handling

The most defining difference is their intended power output. The D77 was a workhorse built to handle the high currents of EMD's 2,000–3,600 hp locomotives. The D87 was specifically designed for EMD's more powerful 3,500–3,800 hp "50-Series" models.

To accomplish this, its continuous current rating was increased from the D77's 1,050 amps to 1,170 amps—an 11% advantage. This higher current capacity directly contributed to greater tractive effort.

Design and Construction Improvements

The D87 incorporated internal modifications that set it apart from the D77:

The Experimental "D77X"

The D77X was a high-capacity variant of the D77 used exclusively in the experimental DDA40X "Centennial" locomotive. Built in 1969–1971 for Union Pacific, this was the largest and most powerful diesel-electric locomotive ever built. It used eight D77X traction motors to manage its immense 6,600 hp output (from twin 16-645E3A engines). The D77X likely incorporated internal upgrades from the standard D77 to handle this massive power.

Common Mechanical "DNA" (Similarities)

Despite their differences, the motors share significant commonality, underscoring EMD's strategy of evolutionary design. They are built on the same basic "D-series" architecture, with many parts being interchangeable.

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