>>1223345UP had this idea that locomotives largely cost the same to maintain, regardless of size, and that by having fewer, larger locomotives they could effectively slash their maintenance budgets compared to using more, smaller locos. This was especially important as the big mountain passes in Wyoming that the UP crossed needed major power to haul trains over.
It's the philosophy that birthed the Big Boy and the Challenger, so that they could use one giant loco instead of two normal sized one, and it further led to the GTELs which were meant to accomplish the same thing. Once bunker C got too expensive for the GTELs to make economic sense, UP experimented with 6000+hp diesels built from siamesed road switcher guts, and solicited bids from Alco, GE, and EMD, all of which were two ~3000hp prime movers each driving 4 independent traction motors and mounted on a single frame. All of them were more or less failures, as they weren't much cheaper to maintain than two road switchers and if one went down you had to scrub an entire train since there were never many built in the first place. The "giant loco" concept was actually a sound one, but it only made economic sense on heavily maintenance intensive designs like steam locos, gas turbines, and early 40s-50s era diesels.
Here's one of EMD's DD40X locomotives, basically a pair of GP40s siamesed together with a pair of prime movers/generators each driving their own 4-motor bogie. Note the walkthrough passageway halfway down the loco between the two independent power modules and the gap between their separate radiators.