The EMD SW1 is a 600-horsepower (450 kW) diesel-electric switcher locomotive built by General Motors' Electro-Motive Corporation (later Division) between December 1938 and November 1953.

661 locomotives of this design were built,[1] with a gap in production between March 1943 and September 1945 due to World War II.

Designed specifically for railroad locomotives, this was a mechanically-aspirated, two-stroke, 45 degree V type, with an 8+1⁄2 by 10 in (216 by 254 mm), bore by stroke, giving 567 cubic inches (9.29 L) displacement per cylinder.

A direct current generator provides power to four traction motors, two on each truck, in a B-B arrangement.

Externally, the two center cab windows over the hood, which were curved to follow the roofline originally, became flat-topped after mid-1950.