I work at a train museum. Locomotives are fundamentally different. They have to have electric traction motors to grab the rails. Weight is unimportant. 125 tons. That's a quarter million pounds, 113,400 kg. Automobiles need every weight-saving trick the engineers can pull off. Such as combination drives.

Also trains don't change speed super often so they can have their prime mover running in the power band all of the time. A car changes speed at lot so its more inefficient to have the prime mover running in its power band constantly to drive the generator.

This is one of the functions of the Chevy Volt - "...its internal combustion engine powers an electric generator to extend the vehicle's range as needed. When the engine is running it may be periodically mechanically linked (by a clutch) to a planetary gear set, and hence the output drive axle, to improve energy efficiency." -- from wikipedia page on the Volt

Yeah, I'd love to know why this hasn't been an option. Seems like the best of both worlds