# ALCO S-2 and S-4
The ALCO S-2 and S-4 were 1,000 horsepower (746 kW) diesel-electric switcher locomotives built by the [American Locomotive Company](https://www.edgepedia.org/american-locomotive-company) (ALCO) and its Canadian licensee Montreal Locomotive Works (MLW). The two models were mechanically near-identical, differing mainly in their trucks: the S-2 rode on ALCO's own "Blunt" trucks, while the S-4 used AAR type A switcher trucks.
| Key fact | Detail |
|---|---|
| Power type | 1,000 hp (746 kW) diesel-electric end-cab switcher, B-B wheel arrangement[1](https://www.thedieselshop.us/DataS2S4m.HTML) |
| Prime mover | ALCO 539, six-cylinder inline, four-stroke, turbocharged (Buchi system), rated 1,000 hp at 740 rpm[2](https://projects.esu.eu/projects/print/S0511) • [3](https://www.schenectadyhistory.org/railroads/manuals/tp107a/) |
| Truck difference | S-2: ALCO "Blunt" 4-wheel trucks; S-4: AAR Type A switcher trucks[1](https://www.thedieselshop.us/DataS2S4m.HTML) |
| Production | S-2: about 1,500 units, 1940 to June 1950; S-4: about 650 to 800 units, 1949 to 1957 (MLW) or into 1961 (ALCO), sources differ[1](https://www.thedieselshop.us/DataS2S4m.HTML) • [4](https://www.american-rails.com/705.html) |
| Tractive effort | Starting 69,000 lbs (S-2) and 72,400 lbs (S-4) at 30%; continuous 29,200 lbs at 6 mph (S-2) and 34,000 lbs at 8 mph (S-4)[1](https://www.thedieselshop.us/DataS2S4m.HTML) |
| Weight and speed | 229,700 lbs; top speed 60 mph[1](https://www.thedieselshop.us/DataS2S4m.HTML) |
| Electrical gear | GE GT553 (S-2) or GT563 (S-4) main generator; four GE 731 or 731D traction motors[1](https://www.thedieselshop.us/DataS2S4m.HTML) |
## Design and the 539 engine
The S series replaced ALCO's earlier high-hood HH-series switchers, whose McIntosh & Seymour 531 engine was superseded by the 539 in 1939; the S-series units entered ALCO's catalog in 1940.[5](https://utahrails.net/articles/up-sw-other.html) The 539 was developed in response to a perceived drawback of ALCO's switchers, a restricted field of vision caused by the high hood, at a time when Baldwin and EMC were offering lower-hood designs trainmen could see over.[2](https://projects.esu.eu/projects/print/S0511) The S-2 was the turbocharged version of the 660 hp S-1, which used the same engine without a turbocharger.[6](https://archive.atlasrr.com/HOLoco/arc-hos2s4.htm)
The 539T was equipped with the Buchi turbocharger, built under license by the Elliott Manufacturing Company of Jeannette, Pennsylvania. The first 539T engines were installed in S-2 switchers and DL-105 passenger locomotives built in September 1940.[2](https://projects.esu.eu/projects/print/S0511) The enginemen's manual describes the engine as a six-cylinder, single-acting, four-stroke design developing full rated horsepower at 740 rpm, with the 1,000 hp version turbocharged on the Buchi system.[3](https://www.schenectadyhistory.org/railroads/manuals/tp107a/)
## S-2 versus S-4
The S-2 was built between August 1940 and June 1950, with 1,502 completed according to the standard account; other rosters give 1,462 or 1,463 units, a difference that reflects how Canadian-built and special-order units are counted.[1](https://www.thedieselshop.us/DataS2S4m.HTML) • [7](https://www.american-rails.com/4512.html) It was ALCO's best-selling small switcher, and the S-2 and S-4 together outsold the lower-powered S-1 and S-3 by more than 1,400 units.[7](https://www.american-rails.com/4512.html) • [4](https://www.american-rails.com/705.html)
The S-4 was simply an S-2 with AAR trucks instead of Blunt trucks, using the same 1,000 hp turbocharged 539 engine.[8](https://cnrha.ca/s-4/) Minor carbody changes included relocating the bell from behind the exhaust stack to the front of the hood unless a railroad requested otherwise.[4](https://www.american-rails.com/705.html) The S-4 offered slightly more tractive effort, 72,400 pounds starting and 34,000 pounds continuous against the S-2's 69,000 and 29,200.[4](https://www.american-rails.com/705.html)
Production totals and end dates for the S-4 vary by source. American-Rails gives 782 units with production completed in 1960,[4](https://www.american-rails.com/705.html) while the Trains.com forum and a data sheet list production running to January 1961.[9](https://forum.trains.com/t/alco-switcher-differences-s-2-vs-s-4/288235) • [1](https://www.thedieselshop.us/DataS2S4m.HTML) The first MLW S-4 entered production in June 1949 as Canadian Pacific #7099.[4](https://www.american-rails.com/705.html)
## Wartime and postwar sales
The S-2 benefited from World War II production restrictions. [Union Pacific](https://www.edgepedia.org/union-pacific)'s first diesel switchers were EMD NW2s delivered in 1940, but War Production Board restrictions on railroad use of EMD's 567-model diesel engine, needed for the war effort, forced the carrier to look elsewhere. UP's first S-2s, 19 units numbered D.S.1036-1054, were delivered between September 1943 and February 1944, followed by 35 more between June 1945 and January 1946, for a total of 54, the only S-2s on the railroad.[5](https://utahrails.net/articles/up-sw-other.html) UP later bought 45 S-4s, delivered between June 1955 and March 1956, giving it 99 ALCO switchers against 171 EMD units purchased over the same era.[5](https://utahrails.net/articles/up-sw-other.html)
Canadian railroads were significant buyers through MLW. Between 1941 and 1959 Canadian National acquired 317 ALCO and MLW switchers, including 51 S-2s and 128 S-4s, preferring the turbocharged 539 and later 251 prime movers.[10](https://cnrha.ca/mlw-switchers/) In 1955 ALCO and MLW changed from riveted to welded carbodies, and MLW repositioned the number boards and sand filler lids, producing two recognized subtypes, the early and late MLW S-4.[8](https://cnrha.ca/s-4/) MLW's first production diesel locomotive was an S-2, Canadian Pacific 7077 built in May 1948, now preserved at Exporail.[11](https://bytownrailwaysociety.ca/images/files/Preserved-QUE-NB-PE-NS-NL.pdf)
## Preservation
Both models survive in notable numbers. The New York, Susquehanna & Western ALCO S-2 #206 at the Maywood Station Museum in New Jersey was listed on the [National Register of Historic Places](https://www.edgepedia.org/national-register-of-historic-places) in 2010.[12](https://npgallery.nps.gov/AssetDetail/NRIS/09001072) The Pacific Northwest Chapter of the NRHS holds NPTCo S-2 No. 36, an E-1540 built in Schenectady in 1943, at Powerland Heritage Park.[13](https://www.pnwc-nrhs.org/Trainmaster2021/TM-2021-08.pdf) American-Rails describes the S-4 as one of the best-preserved ALCO diesel models, with more than a dozen units preserved or in operation at museums and tourist lines.[4](https://www.american-rails.com/705.html) A considerable number of both models operated well into the 21st century.[14](https://www.loco-info.com/view.aspx?id=15885&t=)
## References
1. [Alco S-2 and S-4 Data Sheet, The Diesel Shop](https://www.thedieselshop.us/DataS2S4m.HTML)
2. [Bulletin: ALCO 6-539T, ESU project S0511](https://projects.esu.eu/projects/print/S0511)
3. [Alco-General Electric Diesel-Electric Switcher, Manual for Enginemen TP 107A, Schenectady Digital History Archive](https://www.schenectadyhistory.org/railroads/manuals/tp107a/)
4. [Alco "S4" Locomotives: Specs, Roster, Data Sheet, American-Rails.com](https://www.american-rails.com/705.html)
5. [Union Pacific ALCo, Baldwin, F-M, and Other Switchers, UtahRails.net](https://utahrails.net/articles/up-sw-other.html)
6. [Atlas HO S-1, S-2 & S-4 Locomotives, Atlas archive](https://archive.atlasrr.com/HOLoco/arc-hos2s4.htm)
7. [Alco "S2" Locomotives: Specs, Roster, Data Sheet, American-Rails.com](https://www.american-rails.com/4512.html)
8. [S-4, Canadian National Railways Historical Association](https://cnrha.ca/s-4/)
9. [Alco Switcher Differences (S-2 vs. S-4), Trains.com Forums](https://forum.trains.com/t/alco-switcher-differences-s-2-vs-s-4/288235)
10. [MLW Switchers, Canadian National Railways Historical Association](https://cnrha.ca/mlw-switchers/)
11. [Preserved Locomotives, Quebec, Bytown Railway Society](https://bytownrailwaysociety.ca/images/files/Preserved-QUE-NB-PE-NS-NL.pdf)
12. [New York, Susquehanna & Western Railroad ALCO Type S-2 Locomotive #206, National Register of Historic Places, NPGallery](https://npgallery.nps.gov/AssetDetail/NRIS/09001072)
13. [ALCO S2 at Powerland, The Trainmaster, Pacific Northwest Chapter NRHS, August 2021](https://www.pnwc-nrhs.org/Trainmaster2021/TM-2021-08.pdf)
14. [ALCO S-1 to S-6, loco-info.com](https://www.loco-info.com/view.aspx?id=15885&t=)
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*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Rail transport › Rail vehicles and rolling stock › Diesel, electric, and wartime locomotives › North American diesel and electric locomotives › ALCO diesel locomotive classes*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license