Built by the American Locomotive Company (Alco) the low-hood S-2 was introduced in 1940 to replace Alco’s earlier high-hood switchers. The 1000 horsepower S-2 was a turbocharged version of the S-1. There were 1,502 S-2’s sold to North American Railroads.
Alco introduced the S-4 in 1950. The S-4 like the earlier S-2 was a 1000 horsepower switcher. Some 636 S-4’s were built for North American railroads.
The primary quick spotting difference between the S-1/S-2 and the S-3/S-4 can be seen in the trucks. The S-1/S-2 has Blunt trucks, while the S-3/S-4 has AAR-style trucks.
The versatility of the S-2’s and S-4’s were evidenced by their service in mainline railroads, shortlines, and industrials.
