Union Pacific introduced the 4-6-6-4 Challenger type in 1936 when it received 15 engines from Alco. UP eventually owned 105 of the 252 4-6-6-4s produced; No. 3937 is from the road’s final order, delivered in 1944.
Art Stensvad photo
UP Challenger type
By Angela Cotey | October 20, 2018
| Last updated on January 4, 2021
