The world’s largest operating steam engine is currently Union Pacific Railroad’s (UP) locmotive 4014 (ALCO, 1941/69585) in the USA. It belongs to the UP class 4000, nicknamed ‚Big Boy‘. This is one of the largest steam engine types ever build (see below). In total 25 of those engines were built in 1941 (20) and 1944 (5). They severed on UP’s network mainly across the Rocky Mountains until there were withdrawn in 1962.
UP restored the 4014 ‚Big Boy‘ to celebrate the 150th annniversary of the completion of the transcontinental rairoad in 2019. It operates today for PR purposes at public and employee events. It serves as a national symbol and people gather in sometimes pretty large crowds to see it move by or at one of the frequent maintenance stops. UP supports this by promoting information about its schedule and movement. There is no ambition by UP to run historic trains similar to what heritage railways do.
Photographed July and October 2024 and July 2026.
To illustrate the immense size of the class 4000 here a comparison with some of the largest engines of SNCF, LNER and DB from the same period:
| UP Class 4000 | SNCF Class 241 P | LNER A4 | DB Class 01.10 | |
| Build | 1941, 1944 | 1948-52 | 1935-38 | 1939-40 |
| Number | 25 | 36 | 35 | 55 |
| Wheel Base | 4-8-8-4 | 4-8-2 | 4-6-2 | 4-6-2 |
| Weight (t) | 540 | 216 | 170 | 196 |
| Lenght (m) | 40,4 | 27,1 | 21,7 | 24,1 |
| Weight/m (t/m) | 13,3 | 7,8 | 7,8 | 8,1 |
| Power (HP) | 6500 | 4000 | 2200 | 2350 |
| HP/t | 12,3 | 18,5 | 12,9 | 12 |
| Tractive Force (kN) | 600 | 200 | 160 | 156 |
| Max. km/h | 110 (130 design) | 120 | 145 (201 WR) | 140 |
| Ø Driving Wheel (mm) | 1727 | 2020 | 2032 | 2000 |
| Boiler (bar) | 21 | 20 | 17 | 16 |
| Cylinder | 4 | 4 compound | 3 | 3 |
| End of Service | 1962 | 1969 | 1966 | 1975 |
| Preserved/Operating | 8/1 | 4/1 | 6/3 | 10/2 |