ALCO S4 No. 1825 at San Francisco, CA on August 25, 1967.

(Roger Puta courtesy Marty Bernard, railfan 44, Public domain, via Wikimedia Commons, cropped)



The ALCO S-2 and S-4 are 1,000-horsepower (746 kW) diesel electric switcher locomotives produced by ALCO and Canadian licensee Montreal Locomotive Works (MLW).

Powered by turbocharged, 6-cylinder ALCO 539 diesel engines, the two locomotives differed mainly in their trucks: the S-2 had ALCO "Blunt" trucks; the S-4, AAR type A switcher trucks. A total of 1,502 S-2s were built from August 1940 to June 1950; 797 S-4s were built from June 1949 to August 1957. The S-4 was first produced in Canada, with ALCO production beginning in June 1949.

The S-2 and S-4 were designed as rail yard switchers, meant to replace older, less efficient, and more demanding steam switchers. They were a success, with many remaining in service today.

The locomotives' exterior was styled by ALCO engineer Ray Patten, who used curves in a mild application of Art Deco principles.

The S-2 and S-4 are distinguishable externally from the very similar S-1 and S-3 660 hp (492 kW) switchers in that they have a larger exhaust stack with an oblong base and a larger radiator shutter area on the nose sides. The S-1/S-3 radiator shutter area is taller than it is wide, while the S-2/S-4 radiator area is wider. The larger stack is due to turbocharging. The carbody and cab of late S-2s are nearly indistinguishable from those of S-4s. Hence, a truck swap can cause many to misidentify a unit.



A few S-2s and S-4s are still in service on short line railroads around the United States. Several more are preserved in U.S. and Canadian railroad museums.


ALCO S-2 No. 224 CYDZ (Conrad Yelvington Distributors) in Orlando, FL.

(Meloaraujo, CC BY-SA 4.0 <>, via Wikimedia Commons)



Conrad Yelvington Distributors, an aggregate supplier in Orlando, Florida, owns and operates six S-2 locomotives and one S-4 locomotive. The S-2's include former C&O 5029, B&O 516 Ontario Northland 1202 and 1201, and Seaboard Air Line 1428 and 1431. They are now Conrad Yelvington locomotives 224, 238, 239, 251, 317, and 366, respectively. The S-4, formerly C&O 5105, is now Conrad Yelvington locomotive 365.

The Oil Creek and Titusville Railroad operates S-2s No. 75 and No. 85 on its tourist/freight railroad.

The Toledo, Lake Erie, and Western owns three ALCO S-2 locomotives and one ALCO S-4. TLEW 62, a S-2 purchased in 2012, ex. Delray Cement 62, TLEW 112, a S-2 that was part of the original TLEW roster, now reduced to a parts unit as of 2010, TLEW 5109, a S-4, and the only operating ALCO on the line currently. 5109 recently was repainted into its original Chesapeake and Ohio colors in September 2013.

An ALCO S-2 built in 1946 was serving the Columbia & Reading Railway as No. 2-26 in Columbia, Pennsylvania, during 2019 after first operating on the C&O as No. 5015 and later on six other railroads.

The North Alabama Railroad Museum in Huntsville, Alabama runs one S-2 in regular tourist excursions, Mercury & Chase No. 213. It also owns another S-2, Mercury & Chase No. 484, which returned to service with No. 213 in 2018. The museum also has ex-Santa Fe No. 1534, an ALCO S-4 that is not in service.

The San Francisco Bay Railroad, the short-line railroad for the Port of San Francisco, operates S-2s No. 23 and No. 25 from the San Francisco Belt Line Railroad.

An S-2 of D&RGW heritage survives on the Big South Fork Scenic Railway, as number 102. It was purchased in February 1964 for the Kentucky and Tennessee Railway, and is in operable condition in Stearns, Kentucky. This was one of the diesels that replaced Southern 2-8-2 No. 4501 on the K&T.

Southern Pacific 1474 is in operation, in rotation, at the Orange Empire Railway Museum in Perris, California, pulling a tourist train on weekends.

The Cooperstown and Charlotte Valley Railroad operates a pair of restored ex-Canadian National units S-4 No. 3051 (formerly CN No. 8181) and S-7 No. 3052 (formerly CN No. 8223). In 2017, they acquired the former Concord and Claremont Railroad ALCO S-4 units S-4 No. 102 (formerly D&H No. 3050) and S-4 No. 104 (formerly D&H No. 3036). As of 2020, all but No. 104 were operational on the tourist passenger and maintenance of way services between Milford and Cooperstown, New York. S-7 No. 3052 is thought to be the last S-7 built that is still in operation.

The coal-hauling Beech Mountain Railroad in Alexander, West Virginia, rosters an S-2 (No. 113) and an S-4 (No. 115). Both were built new for Michigan Limestone and Chemical Company.

The Minnesota, Dakota and Western Railway operates five S-2 locomotives, MD&W 16, 17, 18, 19, and 20, which were formerly B&O 512, Y&N 220, Toledo Terminal 103, B&O 500, and Northern Pacific 716, respectively.

In the mid-1960s, Hamersley Iron purchased an S-2 for use in the Pilbara region of Western Australia.


Grafton and Upton Railroad No. 1001, an S-4, rests in Hopedale, MA on 11 May 2008.

(NJPolselli, Public domain, via Wikimedia Commons)



  • Baltimore and Ohio Railroad 9115 has been cosmetically restored at the West Chester Railroad. In 2022 it was repainted to its original B&O paint scheme. It is not currently in service
  • Western Pacific 563, one of two S-4s purchased by that railroad, is today preserved at the Western Pacific Railroad Museum at Portola, California.
  • Erie Railroad S-2 518 is at The Meadville Railroad Depot Museum.
  • New York, Susquehanna and Western Railway S-2 No. 206 sits cosmetically restored at Maywood Station in Maywood, New Jersey.
  • In Muskogee, Oklahoma, at the Three Rivers Museum, a S-2 No. 63-138 sits behind the Midland Valley Station.
  • The Houston Railroad Museum in Houston, Texas, has two S-2s: ex-Santa Fe No. 2350 and ex-Houston Belt and Terminal No. 14.
  • The Gold Coast Railway Museum possesses NASA S-2 No. 1, which was used to switch freight at NASA's Kennedy Space Center.
  • In June 2023, the Western Maryland Rail Heritage Foundation called for donations to relocate an S-2 that was on a siding in Canada.


Sacramento Northern Alco S2 404 at Stockton, California, September 15, 1972. Photographer: D. Ross.

In 1967, Western Pacific went bargain hunting for switchers for the Sacramento Northern and Tidewater Southern.

This unit was built as International-Great Northern 9158, later becoming Missouri Pacific 1053.

(Craig Garver, Public domain,



Type and origin
Power type: Diesel-electric
Builder: ALCO and MLW
Model: S-2 and S-4
Build date S-2: August 1940 – June 1950
Build date S-4: June 1949 – August 1957
Total produced S-2: 1502, Total produced S-4: 797
​• AAR B-B
Gauge: 4 ft 8+1⁄2 in (1,435 mm) standard gauge
Trucks S-2: Blunt
Trucks S-4: AAR type A
Wheel diameter: 40 in (1,016 mm)
Minimum curve: 50° or 118.31 ft (36.06 m)
Wheelbase: 30 ft 6 in (9.30 m)
Length: 46 ft (14.02 m)
Width: 10 ft 2+1⁄2 in (3.11 m)
Height: 14 ft 6 in (4.42 m)
Locomotive weight: 230,000 lb (100,000 kg)
Fuel capacity: 635 US gal (2,400 L; 529 imp gal)
Prime mover: Alco 539T
Engine type: Inline 6 Four-stroke diesel
Aspiration: Turbocharged
Displacement: 1,595 cu in (26.14 L) per cylinder; 9,572 cu in (156.86 L) total
Generator: GE GT 553-A
Traction motors: (4) GE 731
Cylinders: 6
Cylinder size: 12+1⁄2 in × 13 in (318 mm × 330 mm)
Performance figures
Power output: 1,000 hp (746 kW) @ 740 rpm
Tractive effort: 57,500 lb (26,100 kg)
Locale: United States, Canada, Mexico, Australia