Why doesn't every single diesel electric locomotive from the 1940s on have "free" air conditioning? There is this thing they've had since 1930s called absorption refrigerators. They basically use heat to cool an area down. They're mostly found today as propane refrigerator used in RVs. The best part is they're very simple and have no moving parts (maybe a fan to circulate air over the cold coils). A locomotives prime mover generates plenty of heat. Run the hot part of the circuit on the prime mover and the cold part of the circuit in the cab And you basically have an air conditioning system that takes no extra energy (Again except for maybe a small fan) is very simple and has little to break down.

When there's what appears to be a simple and obvious solution that nobody is using, you have to ask yourself why. There must be some reason they didn't go that route.

The answer, in this case, is quite simple. They're not very efficient at all, and regular compressor based cooling is much more efficient. As for free air conditioning? That's what cab windows are for.