The 197,520–203,980 lb (89,600–92,500 kg) units were powered by a six-cylinder diesel engine rated at 660 horsepower (492 kW), and rode on two-axle AAR Type-A switcher trucks in a B-B wheel arrangement.

142 examples of this model were built for American railroads, along with the United States Navy.

In the early 1960s the Reading Company sent all 10 of their VO-660s to General Motors Electro-Motive Division to have them rebuilt to SW900 specifications.

These locomotives received new frames, cabs, and carbodies, and reused only the trucks and batteries from the VO-660's.

It was sold by the Altoona Railroaders Memorial Museum to SMS Lines to be repaired and put back into operation.