The ALCO DL420 was the prototype ALCO S-5

ALCO S-5

The Alco S-5 was a diesel-electric locomotive of the switcher type rated at , that rode on two-axle trucks, having a B-B wheel arrangement....

 switcher

Switcher

A switcher or shunter is a small railroad locomotive intended not for moving trains over long distances but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been...

 diesel-electric locomotive.

It was built in August, 1951 using an early six cylinder 251 engine rated at 800 hp and RS-3

ALCO RS-3

The ALCO RS-3 is a 1,600 hp , B-B road switcher railroad locomotive. It was manufactured by American Locomotive Company and Montreal Locomotive Works from May 1950 to August 1956, and 1,418 were produced — 1,265 for American railroads, 98 for Canadian railroads, 48 for Brazilian and 7...

 subassemblies in its construction. It rode on two-axle trucks

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

, having 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 and then at Schenectady as a shop switcher (#6). The unit was scrapped in the late 1950s.