The Century 855 was designed and built specifically for Union Pacific, which was in need of very high horsepower locomotives for its Overland Route through the Rocky Mountains.

[2] Despite its impressive power output, all three examples of the class were scrapped by early 1972 due to mechanical unreliability.

The C855 rode on four two-axle trucks, grouped in pairs linked by span bolsters, giving a wheel arrangement of B+B-B+B.

[1] Union Pacific also wished to reduce the number of locomotives needed on a consist, to keep in line with their allowable gross weight limit.

They spent their lives in the general freight pool at North Platte; ALCO fitted these units with aluminium wires instead of regular copper (to reduce construction costs), but due to overheating, suffering electrical fires, and constantly shutting down by its twin prime movers led to their early retirement and eventual scrapping by February 1972, after only being in service for less than eight years.