The diesel engine runs in one direction only just like a car engine. But a reversing gear is not needed as it drives a generator. The electric motors driving the wheels go either way equally. Reversal is made by reversing the connections from the generator to the motors.

This is simplified in several ways: alternators instead of generators, alternator output chopped to variable frequency AC, etc. but the basic idea remains the same. The engine is always rotating in one direction only. Electric motors aren't particular.