The reference for locomotives and railcars
Union Pacific class 4000 “Big Boy”
United States | 1941 | 25 produced
No. 4002 in December 1954 in Laramie, Wyoming
No. 4002 in December 1954 in Laramie, Wyoming
A. Poschman / collection Taylor Rush

After the success of the Challenger, the Union Pacific planned to have only one locomotive haul express freight trains on Wasatch Grade and Sherman Hill in the future. To haul trains of 3,600 short tons and over it was calculated that an adhesive mass of 540,000 pounds would be required. The grate area should be large enough to be able to develop high power even with inferior coal. This required eight coupled axles and also four carrying axles in order to be able to carry a sufficiently strong boiler. At the same time, the locomotives should be designed for a speed of 80 mph

The locomotive developed in this way was the heaviest, but not the most powerful, conventional steam locomotive of all time. Like most modern American articulated locomotives, they had four cylinders with simple steam expansion and were therefore no true Mallets. The running numbers started from 4000, in fact the class was given the designation 4884 due to the lack of a name for the wheel arrangement. The Union Pacific first wanted to officially call it “Wasatch”, but the unofficial name “Big Boy” prevailed. In 1941, 20 class 4884-1 locomotives were built and in 1944 five more class 4884-2 locomotives.

In order to allow encounters between two Big Boys, the gap between the tracks had to be increased in many curves. The chassis had no problems with tight corners. In practice, they also pulled trains of 6,000 short tons and often reached their permitted speed of 70 mph with lighter trains. The number 4005 was converted to oil firing, but was converted back to coal firing after a short time. They later received an extension on the tender that increased capacity from 28 to 32 short tons

In the 1950s, the Big Boys were replaced by the gas turbine locomotives. The last commercial journey with a freight train took place in 1959. The 20 locomotives of the first series all achieved a mileage of just over a million miles. A total of eight Big Boys still exist today. The 4014 was the only one to be rebuilt and has been operational again since 2019. This makes it the most powerful operational steam locomotive today.

Axle config4-8-8-4 (Big Boy (Mallet)) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length132 ft 9 5/16 in
Wheelbase72 ft 5 1/2 in
Fixed wheelbase18 ft 3 in
Total wheelbase117 ft 7 in
Service weight762,000 lbs772,250 lbs
Adhesive weight540,000 lbs545,200 lbs
Total weight1,189,500 lbs1,208,750 lbs
Axle load67,800 lbs
Water capacity24,000 us gal25,000 us gal
Fuel capacity56,000 lbs (coal)
Power sourcesteam
Indicated power7,150 hp (5,332 kW)
Estimated power6,900 hp (5,145 kW)
Optimal speed34 mph32 mph
Top speed70 mph
Starting effort135,375 lbf
Power Plant
Driver diameter68 in
Boiler pressure300 psi
Expansion typesimple
Cylindersfour, 23 3/4 x 32 in
Grate area150.3 sq ft150 sq ft
Firebox area704 sq ft720 sq ft
Tube heating area5,185 sq ft5,035 sq ft
Evaporative heating area5,889 sq ft5,755 sq ft
Superheater area2,466 sq ft2,043 sq ft
Total heating area8,355 sq ft7,798 sq ft
Calculated Values
steam locomotive
Otto Jabelmann
last changed: 03/2023

We use cookies to save the following settings:

  • selected navigation structure
  • selected language
  • preferred units
  • spelling of railway company names

If you refuse the use of cookies, the settings will only be retained for the current session and will be reset to the default values the next time you visit the site.

Display of units

Here you can set the desired unit system for the technical data.

  • Metric: Lengths in meters, weights in tonnes, and volumes in cubic meters
  • Imperial (UK): Lengths in feet/inches, weights in long tons and volumes in imperial gallons
  • Imperial (US): Lengths in feet/inches, weights in pounds, and volumes in US gallons
  • Individual: Depends on the country of origin of each locomotive
Operator names

Here you can set the display of railway company names.

  • Short: Abbreviation or short form of the name
  • Standard: commonly used name, partially translated to German
  • Complete: full name in local language