How a Diesel-Electric Locomotive Works (YouTube)
youtube.com
youtube.com
i only watched the first 30 seconds but was interested in the efficiency number i heard - the train was 3 or 4 times more efficient than a truck semi.
i would have guessed it was 30 or 40 or 300 or 400 times more efficient.
In terms of being 300 more times efficient -- converting mechanical energy to electricity to drive wheels is always less efficient than direct drive because there is an efficiency loss in going from mechanical to electric back to mechanical energy. So this is about what is a more efficient transmission, because there is also loss if you were to use a mechanical transmission. For trains, mechanical transmissions would be infeasible at these power levels and for such a large number of wheels. That is why electricity is added to the mix -- it's really used as a transmission replacement.
The reason why diesel-electric can be more efficient than a semi is because the huge diesel engine is more efficient than the smaller diesel engine on a semi. It has nothing to do with using the electricity=based transmission on the locomotive versus a mechanical transmission on the semi.