Diesel Locomotives (USA): History, Builders, Models & How They Work

Last revised: August 13, 2026

Diesel locomotives—technically diesel-electrics—came into widespread mainline use after Electro-Motive Corporation (later General Motors’ Electro-Motive Division) proved passenger power with the EA/EB family (B&O tests often dated to 1937) and, more decisively, freight power with the FT demonstrator of 1939–40. The idea is older still: experimental and switching diesels date to the World War I era, and internal-combustion railcars even earlier. Steam remained the preferred mainline choice for years; the FT tour and the postwar flood of F-units, Geeps, and SDs finished the conversion.

Today’s newest units—from Progress Rail (EMD heritage) and Wabtec (GE Transportation heritage)—are heavily computerized, monitoring almost every aspect of the machine in service. Models such as the SD70ACe-T4 and ES44AC family have changed how engineers work. The core idea has not: a diesel engine generates electricity that drives traction motors in the trucks.

This hub explains how a diesel-electric works, traces early development from McKeen cars and boxcabs through the FT breakthrough, covers competitive dieselization and why EMD won, and provides directories of builders and models linked to detailed American-Rails.com profiles—the same spirit as our History and Fallen Flags hubs: keep the research map, improve navigation.

On this page: How they work · At a glance · Timeline · Early examples · FT demonstrator · Why EMD won · Builders · Model directories · Types & special topics · 21st century · Related reading

Santa Fe F7A 261-C freight south of Denver Castle Rock Colorado April 15 1965

Santa Fe F7A #261-C (early chicken-wire grill variant) and other Fs lead freight south out of Denver near Castle Rock, Colorado, April 15, 1965. American-Rails.com collection.

How a Diesel-Electric Works

A common misconception is that the diesel engine turns the wheels directly. It does not. The diesel engine is the prime mover: the energy it creates drives a main generator (or alternator), which feeds electricity to traction motors in the trucks. Those motors turn the axles. The engine itself has no mechanical connection to the motion of the locomotive.

In essence the machine is an electric locomotive carrying its own power plant. A pure electric draws energy from overhead catenary or third rail; a diesel-electric generates that energy on board. The distinction matters for understanding everything from fuel logistics to why multiple-unit consists work so well: electrical control systems can coordinate several locomotives under one engineer.

Modern units monitor hundreds of parameters, operate routinely in multiple-unit consists, and often run distributed power mid-train or at the rear. Cab electronics have changed how engineers work—but the diesel-to-electric-to-wheels chain remains the core idea, from the 1920s boxcabs to today’s Tier 4 road freights.

Rio Grande F7A 5691 Denver Colorado 1970s

Rio Grande F7A #5691 in Denver, Colorado, during the 1970s. American-Rails.com collection.

At a Glance

Topic	Notes
First U.S. rail diesel experiments	Southern Pacific test-bed gear circa 1904–05 with GE electrical equipment (Solomon, GE Locomotives); GE GM-50 experiments ~1917; limited early production switchers
Landmark early production switcher	Alco-GE-Ingersoll-Rand 60-ton boxcab; CNJ #1000 (1924/25 service-era classic)—often cited as first commercially successful diesel switcher
Early IC railcars	McKeen gasoline cars; later gas-electric “doodlebugs”; branch-line precursors to diesel power
First mainline diesel (often cited)	B&O / Electro-Motive boxcab #50 (1935) for mainline service experiments
Passenger streamliner diesels	EMC EA/EB and related E-units; B&O EA/EB tests often dated to 1937 in company lore
Freight diesel breakthrough	EMC/EMD FT demonstrator national tour 1939–40 (~5,400 hp four-unit A-B-B-A set #103); ~83,764 miles on 20 Class Is in 35 states (Morgan, Trains, Feb. 1960)
Major historic builders	EMD (GM), GE, Alco, Baldwin (later BLH), Fairbanks-Morse; plus smaller industrial builders
Shakeout	Baldwin/FM gone by early 1960s era; Alco locomotive production ends 1969; GE re-enters full line 1959 (Universal) and later overtakes EMD
Today’s U.S. new-build freight focus	Progress Rail (Caterpillar; EMD heritage) and Wabtec (GE Transportation heritage)
Most powerful production diesel (classic claim)	EMD DDA40X “Centennial” (~6,600 hp)—unique dual-engine design
Most important freight model (historical)	EMD FT and F-unit successors that sealed dieselization

Sources for table: Brian Solomon (GE Locomotives, The American Diesel Locomotive, Electro-Motive E Units and F Units); Jim Boyd, The American Freight Train; David P. Morgan, “The Diesel That Did It,” Trains (February 1960); builder literature and American-Rails.com model profiles.

Timeline (Selected Milestones)

Year	Event
1904–05	UP/McKeen self-propelled gasoline railcar work; SP diesel-electric test-bed with GE gear (Solomon)
1905 / 1908	McKeen “Windsplitter” M-1 (Omaha); McKeen Motor Car Company production from 1908
1917–18	GE experimental GM-50 V-8 diesel; limited 225-hp production (few sales: Jay Street #4, Baltimore, U.S. Army)
1923–25	Alco-GE-Ingersoll-Rand 60-ton boxcab; CNJ #1000 becomes the classic early diesel switcher story
1930s	EMC switchers; B&O boxcab #50 (1935); streamliner-era E-units and related passenger power
1939–40	EMC FT demonstrator #103 national tour; freight diesel credibility established
WWII	War Production Board constraints slow full dieselization; demand still builds
1945–60	Postwar F-unit boom; Geeps and road-switchers; steam exits Class I main lines
1959+	GE Universal (“U-boat”) series launches GE as full locomotive builder
1969	Alco ends U.S. locomotive production
1980s	GE market share overtakes EMD
1990s–2010s	AC traction standard for new high-hp freights; EPA tier emissions drive redesign
2005+	EMD sold out of GM; later Progress Rail / Caterpillar ownership; GE Transportation becomes Wabtec
2010s–2020s	Tier 4 freights, comfort cabs, distributed power routine; battery/hybrid experiments for switching; Progress Rail & Wabtec duopoly for new Class I freight power

Early Examples: Boxcabs, McKeen Cars & Switchers

According to Brian Solomon’s GE Locomotives, diesel-electric ideas appeared on Southern Pacific in 1904–1905 with GE supplying generator and electrical equipment. In 1917 GE tested an experimental unit with a GM-50 V-8 diesel of its own design. Formal production of 225-hp machines followed; sales were few—examples included Jay Street Connecting Railroad #4, a unit for Baltimore, and one for the U.S. Army. Early commercial failure, not the end of the idea.

Undeterred, GE partnered on a new design in 1923 using Ingersoll-Rand prime movers (~300 hp) and Alco carbody construction. Jim Boyd notes in The American Freight Train that the 60-ton boxcab finished in 1924 proved successful and sold to the Central Railroad of New Jersey as #1000, spending years switching around New York. Ironically, despite pioneering work, GE did not launch a full standalone locomotive product line until the late 1950s Universal (“U-boat”) series. Until then it partnered with Alco on electrical gear (traction motors and more) while also building electrification systems—and while Electro-Motive took the volume market.

The early boxcabs are often credited as the first rail diesels, but the first internal-combustion railcars were earlier. William McKeen, Union Pacific’s chief of motive power, developed the concept after Edward Harriman approached him in 1904 about self-powered cars for lightly patronized branches (Solomon, The American Diesel Locomotive). After tests at the Electric Railway Test Commission in St. Louis, an aerodynamic design with a ~100-hp Riotte gasoline engine became UP’s M-1 “Windsplitter,” completed at Omaha shops in March 1905. A refined longer car with a marine engine (roughly double the horsepower) impressed Harriman enough to authorize the McKeen Motor Car Company in 1908.

Early McKeen sales success flattened by about 1912 as reliability issues mounted. GE, Electro-Motive, and others later produced more durable gas-electric “doodlebugs” that pointed toward diesel switchers and road power. See Doodlebugs and RDCs.

Usurping steam on the main line was another matter. Baltimore & Ohio, always a technological trailblazer, is often cited for first mainline diesel use with Electro-Motive boxcab #50 in 1935. Electro-Motive later wrapped a rugged GM 567 prime mover in a sleek carbody—the path from boxcab experiment to F-unit freight power.

Santa Fe F7A 335-L Ardmore Oklahoma February 27 1974

Santa Fe F7A #335-L eases out of the siding with a southbound freight at Ardmore, Oklahoma, crossing 5th Avenue, February 27, 1974. American-Rails.com collection.

Rio Grande SW1200 138 Salt Lake City Utah circa 1975

Rio Grande SW1200 #138 switching in Salt Lake City, Utah, circa 1975. Robert Eastwood, Jr. photo. American-Rails.com collection.

Erie Lackawanna Alco HH600 323 Jersey City June 23 1964

Erie Lackawanna HH600 #323, still in DL&W paint (built as #406), Jersey City, New Jersey, June 23, 1964. DL&W owned eight of these early 600-hp Alco switchers (#401–408, 1933–34; #401 was originally Alco demonstrator #603). Alco never officially named the type; railfans later adopted “HH” for High Hood. American-Rails.com collection.

The FT Demonstrator: Real Beginning of the Diesel Age

Diesels did not conquer American railroading in one night, but one event came closer than any other: the 1939–1940 national tour of Electro-Motive’s FT freight demonstrator set. Before the FT, diesels were widely accepted for switching and some passenger work but unproven for heavy mainline freight. After the FT, that skepticism largely collapsed.

Electro-Motive Corporation (soon GM’s Electro-Motive Division) designed the FT for freight from the start. The four-unit demonstrator—A-B-B-A set #103—produced about 5,400 horsepower and showcased multiple-unit control, a freight-worthy carbody, and electrical systems suited to sustained heavy duty. From November 1939 the set toured the country. As Brian Solomon notes in Electro-Motive E Units and F Units, it proved itself again and again—including on Southern Pacific’s Tehachapi Grade, where it outperformed SP and Santa Fe steamers (AT&SF held trackage rights in that territory).

According to David P. Morgan’s classic Trains article “The Diesel That Did It” (February 1960), during trials the locomotives traveled about 83,764 miles on twenty Class I railroads in thirty-five states over an eleven-month period. Results were decisive. Railroads watched the FT handle tonnage that would have demanded more steam power, with far less terminal time and one crew controlling all units. Santa Fe ordered heavily; others followed.

World War II and War Production Board restrictions constrained full conversion, but the postwar F-unit boom confirmed what the demonstrator had shown: diesel-electrics were ready for the hardest freight assignments. The FT did not invent the diesel locomotive—boxcabs, switchers, and passenger units had already proved the technology. What it proved was that diesel could replace steam in the core business: heavy freight over long distances at competitive cost and superior availability. Everything that followed—F3, F7, GP7, and the road-switcher revolution—rested on the credibility the FT earned.

It would take diesels nearly fifty years to equal the raw horsepower of a single steam locomotive at technological peak. They made up for it elsewhere: far less maintenance and heavy shopping, longer runs between fuel stops, more time on the road earning revenue, and MU control that let one engineer command several units. As Solomon notes, while Alco, GE, and Ingersoll-Rand spearheaded early diesel development, it was General Motors / Electro-Motive that successfully mass-produced them for mainline service—durable mechanics and the rugged 567 packaged in an attractive carbody.

Profiles: EMD FT · F-series overview · Electro-Motive history · Model 567.

Soo Line SD40-2 F7B GP9 Stevens Point Wisconsin April 20 1974

Soo Line SD40-2 #760, F7B #2202-C, and a GP9 at the sanding rack, Stevens Point, Wisconsin, April 20, 1974. American-Rails.com collection.

Why EMD Won—and the Others Lost

Postwar dieselization is often told as pure technology. It was also industrial competition. By the late 1950s Electro-Motive Division of General Motors dominated so thoroughly that rivals were gone or marginal. The outcome was not inevitable—different corporate decisions or earlier technology bets might have left more independents alive—but the shape of today’s market still reflects that shakeout.

EMD’s advantages were substantial: standardized mass production when many roads still expected custom engineering; the reliable, improved 567-series engine; strong parts and training support; and the performance reputation built by the FT and successors. Fleet commonality simplified maintenance and crew training. Roads that standardized on EMD gained real operating economies.

Alco entered with solid engineering credentials and produced successful road-switchers—the RS series and later Century models—but never matched EMD’s volume or early reliability reputation. Baldwin, once king of steam, struggled to adapt culture and product; its diesels never gained broad acceptance. Fairbanks-Morse brought an innovative opposed-piston engine (marine roots into the 19th century) but stayed niche and left the locomotive market after roughly a decade (the company itself continues in other businesses). Lima, after joining Baldwin as Baldwin-Lima-Hamilton, exited locomotives. Alco was the last of the traditional independents, ending U.S. locomotive production in 1969.

GE re-entered strongly with the Universal series (1959+). Early U-boats sold commercially but earned a mixed reliability reputation (many had relatively short service lives). GE refined designs through Dash-7 and Dash-8 lines until it overtook EMD in market share by the 1980s—setting up today’s Wabtec vs. Progress Rail duel. EMD never fully recovered its top spot after that shift; General Motors sold the business in 2005, and the corporate division was later dissolved under Progress Rail (Caterpillar), which still uses the EMD name for marketing. A testament to first- and second-generation models endures: units built more than half a century ago still work revenue service on short lines and industrials.

During the high years of dieselization, enthusiasts could see power from four major builders plus vivid railroad paint schemes—cab units, early road-switchers, and company colors in every direction. That diversity narrowed into a duopoly that has lasted, with ownership changes, into the present day.

Baltimore and Ohio E9A 1457 Ivy City Washington DC December 1963

Baltimore & Ohio E9A #1457 (originally #40) in Sunburst livery at Ivy City, Washington, D.C., December 1963. American-Rails.com collection.

Builders (Company Histories)

Diesels have come a long way from the EMC and Alco-GE-IR boxcabs of the 1920s. Today’s models from Wabtec Freight and Progress Rail carry comfort cabs, climate control, and advanced electronics. In the early diesel era there were more than two builders; after Electro-Motive opened the market, Alco, Lima, Baldwin, Fairbanks-Morse, and later GE all competed. The old steam “Big Three” never fully made the transition.

American Locomotive Company (Alco) · Montreal Locomotive Works (Alco’s Canadian arm)
Baldwin Locomotive Works · Lima · Whitcomb
Fairbanks-Morse · Canadian Locomotive Company
General Electric · Wabtec Freight (GE Transportation successor)
Electro-Motive (EMC/EMD) · General Motors Diesel (Canada) · Progress Rail (current EMD owner)
Davenport · Plymouth (industrial / small builders)
Morrison-Knudsen (rebuilds and specialty power)

Model Directories by Builder

Jump into the catalogs of detailed model articles on this site. These directories preserve the full research map from the original diesel hub—Alco through EMD, plus Canadian and specialty designs.

American Locomotive Company (Alco)

The Experimental "Black Maria"
The FA Series
The PA Series
The Century Series
Early DL Design
RS-1
RSD-1
EMD/Alco MRS-1
RS-2
RSC-2
RS-3
RSC-3
RSD-4
RSD-5
RSD-7
RS-11
RSD-12
RSD-15 "Alligator"
RS-27
RS-32
RS-36
C-415
C-420
C-424
C-425
C-430
C-628
C-630
C-636
C-643DH Diesel-Hydraulic
C-855
"High Hood" Series
S-1
S-2
S-3
S-4
S-6
T-6 "Transfer"
General Electric/Alco Gas Turbine Design

Montreal Locomotive Works (MLW)

Baldwin Locomotive Works (BLW)

Fairbanks-Morse (FM)

General Electric (GE)

44-Tonner
70-Tonner
The Classic "U-Boat"
U18B "Baby Boat"
U23B
U25B
U28B
U30B
U33B
U36B
U23C
U25C
U28C
U30C
U33C
U36C
U50
U50C
The Joint General Electric/Alco Gas Turbine Design
B23-7
B30-7
B36-7
C30-7
C36-7
"Super 7" Series Rebuild Program
B32-8 Series
B39-8
B40-8/W
C39-8
C40-8/W Series
C40-9/W
C44-9W
AC4400CW
AC6000CW
ES44AC
ES40DC
ES44DC
ES44C4
ET44AC/ET44C4
Genesis Series

Electro-Motive (EMD)

General Motors Diesel (Canada)

GMD-1

Siemens

Unique Designs

Wabtec Freight

Wabtec Freight

Common Designs, Prime Movers & Special Topics

Beyond builder model lists, these pages cover body styles, engines, trucks, branch-line power, and notable paint/events:

Common Designs

Notable Prime Movers

Truck Types

Notable Events & Paint

Unique / specialty carbodies and experiments are listed under Unique Designs in the model directories (B-units, Centennials, tunnel motors, Krauss-Maffei ML-4000, slugs, Draper taper, and more).

Diesel Power in the 21st Century

The mid-century diesel-electric has itself continued to evolve. The principle—diesel engine, generator/alternator, traction motors—is unchanged; almost every major subsystem has been refined or transformed since the 1990s.

AC traction is the most visible change. Beginning in a serious way with EMD’s SD70MAC and GE’s AC4400CW, AC technology offered better adhesion, higher reliability in demanding service, and improved low-speed performance. By the 2010s AC locomotives were standard for new high-horsepower freights.

EPA emissions tiers forced sophisticated engine controls, exhaust after-treatment, and in some cases entirely new engine designs. Newer units produce substantially less nitrogen oxide and particulate matter than predecessors—at the cost of greater mechanical complexity. Computerization altered the engineer’s role: continuous monitoring of hundreds of parameters, automatic performance adjustments, data links to railroad operating systems, and routine distributed power and precision train handling. The newest Progress Rail and Wabtec units are as much rolling computers as traditional locomotives.

Alternative fuels and hybrids mark the next frontier: battery-electric and hybrid experiments for switching and yard service; renewable diesel and other lower-carbon fuels on limited fleets; gensets and low-emission switchers (see Gensets / Green Goats). Full electrification of the North American freight network remains unlikely near-term, but greenhouse-gas pressure is shaping research and limited deployment.

Builders today: Progress Rail (Caterpillar; EMD designs) and Wabtec (GE Transportation heritage) supply the bulk of new Class I freight power; Siemens and others compete in passenger (e.g., Charger series). The competitive field remains essentially a duopoly for new high-horsepower freights, but corporate ownership and technological priorities have shifted with the times. Diesels have come a long way from the EMC and Alco-GE-IR boxcabs of the 1920s—comfort cabs, climate control, advanced electronics—but they still answer the question the FT asked in 1939: can this machine do the work of steam better?

Steam locomotives · Electric locomotives · Locomotive history
Streamliners · Railroad history · Fallen flags
EMD · GE · Alco · Baldwin · Fairbanks-Morse · Wabtec
Class I railroads · Railroad jobs · Home

Sources

Solomon, Brian. The American Diesel Locomotive; GE Locomotives; Electro-Motive E Units and F Units; related builder studies
Boyd, Jim. The American Freight Train (early boxcab / CNJ #1000 context)
Morgan, David P. “The Diesel That Did It.” Trains Magazine, February 1960 (FT demonstrator mileage and tour scope)
Builder and railroad literature on EMC/EMD FT demonstrator tours (1939–40) and postwar F-unit dieselization
Competitive histories of Alco, Baldwin, Fairbanks-Morse, GE, and EMD diesel lines through the 1969 Alco exit and 1980s GE ascendancy
EPA locomotive emissions standards (tier progression) and industry AC-traction adoption (1990s–2010s)
American-Rails.com model and builder profile pages linked throughout this hub

Diesel hub revised August 2026 (anchors, timeline and glance tables, structured builder/model directories, related reading, MailChimp capture stub) while retaining and bolstering core research on diesel-electric principles, early experiments, McKeen cars, FT breakthrough (including Tehachapi and Morgan tour figures), competitive dieselization, and 21st-century evolution—plus the site’s extensive model-link map for Alco, MLW, Baldwin, FM, GE, and EMD.

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