The ALCO Century 855 was ALCO

American Locomotive Company

The American Locomotive Company, often shortened to ALCO or Alco , was a builder of railroad locomotives in the United States.-Early history:...

's most powerful diesel-electric locomotive and, at the time, the most powerful diesel locomotive ever built. Powered by a pair of 16 cylinder ALCO 251C

ALCO 251

The Alco 251 diesel engine was developed by the American Locomotive Company to replace the 244 and 539. The 251 was developed to be used in diesel locomotives, as a marine power plant in ships and as a stationary power generator.-Development:...

 diesel engines, and rated at 5500 hp, it was ALCO's answer to the EMD DD35

EMD DD35

The EMD DD35 was a diesel-electric locomotive of D-D wheel arrangement built by General Motors Electro-Motive Division for the Union Pacific Railroad and Southern Pacific Railroad. In the early 1960s Union Pacific started asking for a 15,000 hp 3-unit locomotive set to replace the turbines...

 and the GE U50

GE U50

The GE U50 was an eight-axle, diesel locomotive built by GE Transportation Systems. They were twin-engined locomotives, combining two 2,500 hp diesel engines.-Configuration:...

. The C855 rode on four two-axle trucks

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

, grouped in pairs linked by span bolster

Span bolster

A span bolster, in rail terminology, is a beam or frame used to link two trucks or bogies so that they can be articulated together and be joined to the locomotive or railroad car at one rotating mounting point...

s, giving a wheel arrangement

Wheel arrangement

In rail transport, a wheel arrangement is a system of classifying the way in which wheels are distributed beneath a locomotive.. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country...

 of B+B-B+B. The trucks and bolsters were similar to those under UP's earlier turbine locomotives

Union Pacific GTELs

Union Pacific operated the largest fleet of gas turbine-electric locomotives of any railroad in the world. The prototype, UP 50 was the first in a series built by General Electric for Union Pacific's long-haul cargo services. The prototype was introduced in 1948 and was followed by three series...

. Only two A unit

A unit

An A unit, in railroad terminology, is a locomotive equipped with a driving cab, or crew compartment, and the control system to control other locomotives in a multiple unit, and therefore able to be the lead unit in a consist of several locomotives controlled from a single position...

s and one B unit

B unit

A "B" unit, in railroad terminology, is a locomotive unit which does not have a driving cab, or crew compartment, and must therefore be controlled from another, coupled locomotive with a driving cab . The term booster unit is also used. The concept was largely confined to North America...

 were built, all for Union Pacific Railroad

Union Pacific Railroad

The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....

, which had also requested double-engined locomotives from EMD and GE

GE Transportation Systems

GE Transportation, formerly known as GE Rail, is a division of General Electric. The organization manufactures equipment for the railroad, marine, mining, drilling and energy generation industries. It is based in Erie, Pennsylvania. Locomotives are assembled at the Erie plant, while engine...

in order to replace the turbines, which had become uneconomical to operate. Poor performance led to their early retirement and eventual scrapping by February of 1972.