Union Pacific Big Boy at a glance
| Builder | American Locomotive Company (ALCO) for Union Pacific |
|---|---|
| Power type | Articulated steam locomotive |
| Wheel arrangement | 4-8-8-4 |
| Original construction | 25 Big Boys built in 1941 and 1944 |
| Driving wheel diameter | 68 in |
| Boiler pressure | 300 psi |
| Maximum tractive effort | About 135,375 lbf |
| No. 4014 engine + tender length | About 132 ft 9 7/8 in |
| No. 4014 working weight | Engine about 762,000 lb; tender about 427,500 lb |
Why Big Boy existed
Union Pacific needed a locomotive capable of moving heavy freight over the grades of its western main line. The articulated design used two sets of driving wheels under one enormous boiler, allowing high tractive effort while negotiating curves better than a rigid locomotive with the same number of drivers.
What 4-8-8-4 means
In Whyte notation, 4-8-8-4 means four leading wheels, eight driving wheels in the first engine unit, eight driving wheels in the second engine unit, and four trailing wheels supporting the firebox.
Big Boy No. 4014
Union Pacific reacquired No. 4014 in 2013 and restored it in Cheyenne. It returned to service in May 2019. The restored locomotive burns oil rather than the coal it burned during its original working career.
Horsepower vs. tractive effort
Steam-locomotive horsepower changes with speed and operating conditions, so a single horsepower number can be misleading. Big Boy is often more meaningfully described by its approximately 135,375 pounds of maximum tractive effort and its ability to sustain power at speed.
Identification reminder
Locomotives are rebuilt, repainted and modified during long careers. Use several clues together—road number, trucks, hood and radiator details, cab shape and known roster history—rather than relying on paint alone.
Want to know more?
Ask Ask the Conductor about this locomotive, how it works, where it ran or how it compares with another model.
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