The main thing for them is torque. Need a whole buttload of that to get it going. The efficiency of the train then comes from steel wheels on steel rails really. Electric motors is for the torque.

I have understood that idea as with some ships using similar systems is that this allows generators to run at optimal speed for efficiency which is rather tight band when you are looking for few of the last percentage.

With cars the speed range is much larger.

This is correct or close too. The small steel on steel contact surface has so much less friction, that some conversion losses are fine in exchange for massive torque, and less moving parts (The diesel electrics replaced steam locomotives over maintenance cost/operation not pulling power).

At that scale I think ICE engines preferably run at the same RPM continuously, and a gearbox and a clutch would be massive and require a lot of maintenance. The electric motor gives you the huge amount of torque you need to get a train going from stand-still. Not sure what actually tips the scale though.

Cost matter, but the dominate costs are important. The steam engine was more efficient than diesel when they scraped all the steam engines for diesel - steam engines need a lot more labor and so were more expensive despite using less fuel.