> What is commoditized is the engine side of a diesel locomotive, that's your run-of-the-mill CAT, MTU or whatever industrial diesel engine. As long as it fits

> What is commoditized is the engine side of a diesel locomotive, that's your run-of-the-mill CAT, MTU or whatever industrial diesel engine. As long as it fits into the locomotive body and accepts a hydraulic or electric generator at the output, it can be used. IIRC there even were some experiments with gas turbines on French high-speed locomotives, but these had atrocious fuel economy. The diesel prime mover is usually an EMD or Wabtec/GE engine in the US - very few mainline locomotives use anything else. They are medium-speed engines and two-stroke (EMD) or four stroke (Wabtec/GE with some EMD Tier-4), built to be reliable, cheap and nearly indestructible. You can swap other engine makes in, but usually with worse reliability and performance. Outside the US other engine makers are more prevalent, but interestingly US engine designs are the most numerous (except in former Soviet states and China). In the US, newer non-road engines may use Tier-4 MTU or CAT engines for eco friendly power, but they're still the exception and not the rule.