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.
Powered by a pair of 16 cylinder ALCO 251C diesel engines, and rated at 5,500 hp (4,101 kW), it was ALCO's answer to the EMD DD35A and the GE U50. The C855 rode on four two-axle trucks, grouped in pairs linked by span bolsters, giving a wheel arrangement of B+B-B+B. The trucks and bolsters were similar to those under UP's earlier turbine locomotives. Only two A units and one B unit were built, all for the Union Pacific, which had also requested double-engined locomotives from EMD and GE in order to replace the turbines, which had become uneconomical to operate.[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. By consolidating several locomotives into one or two, this allowed them to lower the axle load on the rails, but to stay within the limits placed by the company. 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 it's twin prime movers led to their early retirement and eventual scrapping by February 1972, after only being in service for less than eight years.[3]
