The difference is that diesel electric locomotives don’t have batteries at all (except for starting). The BMW i3 mostly operates off its battery.
The issue is that trains change speed slowly. So a low RPM diesel engine is perfectly suited to generate electricity in this application because you can run at a constant speed for long periods of time. Cars change speed a lot, but that’s one of the things that make ICE’s inefficient. If your ICE is changing RPM all the time, you might as well just mate the power directly to the wheels.