Locomotives / Train Sets

4-8-8-4 (Big Boy)

Steam Locomotive

Series: "Big Boy"

Entry Last Edited: 04/03/2024 | Content ©www.RailRoadJunction.net | The following text is exclusive to this site.
Specifications [+]
Just 25 of the massive Big Boy steam locomotives were built by ALCO for the Union Pacific Railroad in the 1940s.
The 4-8-8-4 was a monster of a locomotive designed, developed, and built by the American Locomotive Company (ALCO) at the behest of carrier Union Pacific (UP). The intent was to produce a new steam-powered loco capable of heavy haul service across the Wyoming wilderness without requiring the use of multiple powered locos for the routes. This led to the 4-8-8-4 which became the only steam locomotive to operate with that particular wheelset arrangement. Just 25 of these massive machines were built from the period spanning 1941 to 1944. The design team was led by one Otto Jabelmann.

The locomotive had a running length of 85.3 feet and a beam of 11 feet. Height reached 16.2 feet. Overall weight of the unit was 760,000lb to 772,000lb with the tender adding another 430,000lb to the mix. The steam-based system was fed by coal pulled from the accompanying tender component of which 28 tons (short) were carried. While steam heat was used for power, a pneumatic system was utilized for braking. With a rated output of up to 7,000 horsepower, maximum achievable speed was 60 miles-per-hour. Tractive effort reached 135,475lb.

The 4-8-8-4 wheelbase gave these locos an exceedingly long shape from bow to stern, the body of the unit being made up primarily of the cylindrical form. At front and rear were the four-wheeled axles with the main mass of the design being supported by sixteen total wheels set up in two groups of eight, these being of larger diameter than the smaller front and rear sets. The driving cab was set at the extreme aft of the design. The tender followed the locomotive immediately aft.

The sole operator of the line became Union Pacific Railroad which operated the design up until 1959 - the last revenue run was recorded on June 21st of that year. UP operated the 1941-era "Class 4884-1" batch across 20 examples (Serial Nos.69571-69590, known to UP under Nos.4000 to 4019). No.4005 was ultimately converted to use fuel oil in 1946 (later modified back to its original form in 1948). The Class 4884-2 batch consisted of the remaining five 1944-era locos (Serial Nos.72777-72781) and UP knew them from Nos.4020 to 4024.

The last unit was officially retired in 1962 and only eight survive today, seven being static display examples and one of the lot having an operational status.

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Specifications [ 4-8-8-4 (Big Boy) ]

American Locomotive Company (ALCO) - USA

United States
1941 - 1944
Production Run

25 units
Production Total
Train driver seating position graphic

Driver Position
26.0m | 85.3 ft
NOTE: May include length of entire train set
345,637 kg
(762,130 lb | 381 US Tons)
Locomotive power source graphical icon
Power Source
Coal-fed steam-based engine.
Engine / Drive Source
7,000 hp
Rated Output Power
97.0 kph
(60.3 mph)
Max Speed
Wheel Arrangement
(Loco Facing Left)

Cab Signals

Safety Systems
Series Variants
4-8-8-4 (Big Boy) - Base Series Name.
Class 4884-1 - First batch of twenty locos in 1941.
Class 4884-2 - Second batch of five locos in 1944.
Operating Countries
United States

National flag of the United States
Customers / Operators:
Union Pacific Railroad
NOTE: Includes both past and present

Image of the 4-8-8-4 (Big Boy)

Image of the 4-8-8-4 (Big Boy)
Image of the first Big Boy locomotive from ALCO, the UP 4000.
Image from the Public Domain.
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