[jcranmer]

Diesel locomotives are closer to electric cars than they are to diesel cars or trucks--the actual traction motors are electric, and the diesel itself is just a diesel generator for power. This means that the main disadvantages of diesel over electric are sourcing fuel, more moving parts to maintain, and some exhaust issues. However, the US railroad network was essentially built out in the steam era, and steam locomotives have much more severe requirements for all of these problems. Going to diesel from steam was already a win.

The problems with electric trains is needing to supply and maintain catenaries, needing to source enough power, and needing to cope with highly variable electric demand (trains use much more power when they hit a steep incline). In the early 20th century, when railroads were replacing their steam locomotives, the rural electrification projects in the US just didn't exist yet, which made electrification of mainline railroad track quite expensive. The few railroads that tried ended up bankrupting themselves in the process.

Another drawback of electrification that's only obvious in hindsight is that installing catenaries makes enlarging the load gauge that much more costly--if the US had mostly electrified its freight track, it likely would have never developed double-stacked container trains (absent in much of Europe since the electrical catenaries provide insufficient clearance), a development which did much to revitalize the railroad industry.