The ALCO RSD-4 was a diesel-electric locomotive of the road switcher type rated at , that rode on three-axle trucks, having an C-C wheel arrangement.

Used in much the same manner as its four-axle counterpart, the ALCO RS-3, though the six-motor design allowed better tractive effort at lower speeds. Due to the inadequate capacity of the main generator, this model was later superseded in production by the ALCO RSD-5

Original owners

Railroad	Quantity	Road Numbers	Notes
Atchison, Topeka and Santa Fe Railway	10	2100–2109
Chicago and North Western Railway	5	1515–1517, 1619–1620
Central Railroad of New Jersey	14	1601–1614
Kennecott Copper Corporation	1	201	Preserved
Utah Railway	6	300–305
Total	36

Preserved units

The only ALCO RSD-4 that has survived is Kennecott Copper Corporation #201. As of 2007, it resides in fully restored condition at the Northwest Railway Museum (formerly known as the Puget Sound & Snoqualmie Valley Railway) in Snoqualmie, Washington, wearing a coat of bright orange paint.