ALCO S-2 and S-4

The ALCO S2 and S4 were switcher diesel locomotives produced by ALCO and Canadian licensee Montreal Locomotive Works (MLW). Basically, the two locomotives differed only in trucks, with the S-2 using ALCO's own Blunt trucks, and the S-4 riding on standard AAR type A switcher trucks. Both were powered by ALCO 539 turbocharged, 6 cylinder diesels. The S-2 was built between April 1940 and June 1950, with a total of 1502 completed, while the S-4 was constructed between August 1950 and January 1961 (MLW until 1957) with total sales of 797. A modified version, the S-7, was built by MLW only; 29 were built between June and August 1957.

Identification

The S-2 and S-4 are distinguishable externally from the very similar S-1 and S-3 660 hp (490 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 mis-identify a unit.

Survivors

A few S4s are still in service on shortline railroads around the United States. Several more are preserved in US and Canadian railroad museums.

Western Pacific 563, one of 2 S4s purchased by that railroad, is today preserved at the Western Pacific Railroad Museum at Portola, California.

References

• Pinkepank, Jerry A. (1973). The Second Diesel Spotter's Guide. Kalmbach Publishing Co., Milwaukee, WI. ISBN 0-89024-026-4.
• Toppan, Andrew et al. Alco/MLW S-2 Roster. Retrieved on December 29, 2005.
• Toppan, Andrew et al. Alco/MLW S-4 Roster. Retrieved on December 29, 2005.