The engine 4014 (ALCO, 1941/69585) of the American Union Pacific Railroad (UP) is the largest steam engine currently operated. It belongs to the UP class 4000, nicknamed ‚Big Boy‘, one of the largest steam engine types ever build. In total 25 of those engines were built in 1941 (20) and 1944 (5) and severed on UP’s network mainly across the Rocky Mountains until 1962.
UP restored the 4014 to celebrate the 150th annniversary of the completion of the transcontinental rairoad in 2019 and operates it today for PR purposes and public and employee events. There is no ambition to run historic trains like heritage railways. The images below were made during the 2024 tours in California, Nevada and Kansas.
Some key data of the class 4000 in comparison with some of the largest engines of SNCF, LNER and DB of the same period to illustrate the immense size of this engine:
| 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 |