The Alco S-5 (DL 421A) was a diesel-electric locomotive of the switcher type rated at 800 horsepower (600 kW), that rode on two-axle trucks, having a B-B wheel arrangement.
This was Alco's second attempt to build a replacement 251-engined switcher to replace the 539-engined S-3 and S-4. Only seven were sold. The demonstrator unit was ALCO specification DL421 and the six production units were ALCO specification DL421A. Alco fitted the 251A engine in its replacement, the S-6 with power output raised by 100 horsepower (75 kW).
