4-8-8-4

A 4-8-8-4 in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck.

Only one model of locomotives has ever used this wheel configuration, and that is commonly known as "Union Pacific Big Boys" after the railroad that operated them.

A similar wheel arrangement exists for Garratt locomotives, but is referred to as 4-8-0+0-8-4 since both engine units swivel.

Eight of these locomotives survive, seven of which are on static public display at various sites in the United States.

Although their wheels were an inch (25.4 mm) smaller than those of the Challengers, they were still able to sustain similar high speeds.