General information

The Class "4000" is a coal burning 4-8-8-4 express freight locomotive of American design, built in 1941 by Alco (American Locomotive Company) at their Schenectady works, and used on the Union Pacific (UP), primarily over the Wahsatch Mountains (Sherman Hill) from Cheyenne to Ogden (westwards ruling grade 1 in 65, eastwards ruling grade 1 in 88). The objective was to take 3,600 ton trains without assistance, at curves of up to 20 degrees (most no more than about 10 degrees). At 10 degrees the front of the boiler would be offset from the centre line by about 2 feet.

When in operation it was found that a tonnage of 4500 tons was acceptable, and after the realignment over Sherman Hill (ruling grade down to 1 in 122) some 6000 tons could be taken by a single locomotive.

The colloquial name "Big Boy" derives from the chalked up name written on the smokebox by an unknow fitter. Union Pacific attempts to call the class the "Wahsatch" failed from the publicity that caught on from the unofficial name, and the name by which they have been known ever since.

A modern articulated design of locomotive it has outside motion, piston valves, Walschaert valve gear, rocking grate, hopper ashpan, water tubes in the firebox, mechanical lubricators, and electric lighting. In addition it also has an automatic stoker.

It has four cylinders with air brakes both for the locomotive and for the train.

Locomotive details


Class Details

Item Value   Item Value
Wheels4-8-8-4 Introduced1941
TrafficExpress freight SuperheatedYes
Boiler pressure300 psi Driving wheels5' 8"
Piston diameter23.75" Piston stroke32"
Valve gearWalschaert Valve typePiston
CylindersFour, outside Tractive effort135,000 lb.
Loco weight344 tons 15 cwt Loco tender194 tons 17 cwt
Loco brakeAir Driving positionRight side
Water capacity24,000 gallons (US) Water capacity20,000 gallons (UK)
Coal capacity28 tons (US) Coal capacity25 tons (UK)