The Alco S-5 was a series of diesel-electric switcher locomotives that were built in 1954 by the American Locomotive Company (ALCO). These locomotives were rated at 800 horsepower (600 kW) with an end-cab design that incorporated the six-cylinder 251A engine. They rode on two-axle trucks and had a B-B wheel arrangement.
ALCO built a demonstrator unit (specification DL421) and then six more (specification DL421A) for the Boston and Maine Railroad (B&M) almost as quickly as it was introduced and were numbered 860-865.
The demonstrator was eventually sold to an industrial operation, which in turn had later sold it to another industrial operation. Four of the six on the B&M eventually had their prime movers swapped out with 251Bs, upgrading them to 1,000 hp. They all remained on the active roster until the 1970s when they were all withdrawn and set aside for scrap.
Six of the seven S-5's have been scrapped leaving only one, B&M No. 864 as the sole survivor. It is currently preserved at the Mad River & NKP Railroad Museum in Bellevue, Ohio.
