The world’s largest steam egine in operation 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 They severed on UP’s network mainly across the Rocky Mountains until there were withdrawn in 1962.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 it today for PR purposes and public and employee events. There is no ambition by UP to run historic trains similar to what heritage railways do. The images here were made during the engine’s 2024 tours in California, Nevada and Kansas.
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 |