Series hybrids do have efficiency advantages in some situations -- for example, they can run in EV mode at any speed, and they can also have more freedom to run the engine at speeds independent of the vehicle speed, which can enable it to be programmed to prefer running it at speeds dictated by efficiency rather than by the drive wheel speed.

The place where they fall behind is at steady state on the highway -- but all of the series hybrid systems on the road have a solution for this problem too! They typically have clutch that engages a one-speed direct drive from the engine to the wheels. This skips the double-conversion losses at highway cruising. Then if you give it some gas to accelerate, the clutch disengages and you go back to full double-conversion again.

Series hybrids, where only electric motors drive the wheels, are becoming more common: Nissan now sells many models under their e-POWER brand.

Efficiency seems to match or exceed conventional hybrids in city driving, and only slightly less efficient for highway driving. And people like the instant torque and the smooth “EV like” driving feel.

I've got a Nissan Epower Kicks. I bought it exactly because the technology was different than standard Hybrid cars.

Here in Mexico there's no infra for fully electric cars, and they are still way too expensive.

The Nissan setup is pretty cool in that the generator is quite small, and the car doesn't need all the mechanical parts of an ICE car. It also gives us the range of a standard ICE car. So far it has been pretty good.

I wonder if fuel efficiency just doesn't matter enough for trains, since they're already so efficient at turning motive energy into motion. Other costs may dominate.

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.

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.