The ALCO DL420 was the prototype ALCO S-5 diesel-electric switcher locomotive.
It was built in August, 1951 using an early six-cylinder 251 engine rated at and RS-3 subassemblies in its construction. It rode on two-axle trucks with a B-B wheel arrangement, and was the first ALCO switcher with a front radiator opening. This one-of-a-kind unit spent its time at the Niskayuna test facility, then at Schenectady as a shop switcher (#6). It was scrapped in the late 1950s.
See also List of ALCO diesel locomotives List of MLW diesel locomotives
References Data from The American Locomotive Company A Centennial Remembrance by Richard Steinbrenner pp. 311-312 photo, 340 photo, 394 photos Rolf Stumpf's ALCO World website switcher specification page