ALCO S-5

The ALCO S-5, specification DL 421A, was a diesel-electric switcher locomotive rated at 800 horsepower that rode on two-axle trucks in a B-B wheel arrangement. It was ALCO's second attempt to build a replacement switcher with the 251 engine to take the place of the 539-engined S-3 and S-4, and only seven were sold: a demonstrator built to specification DL421 and six production units to DL421A. The Boston and Maine was the operator, with fleet numbers 860 to 865 and 909, and one unit is preserved. ALCO fitted the 251A engine in its next replacement, the S-6, which raised the power output by 100 horsepower.