Diesel electric is only used in locomotives (and some big mining equipment) because the realities of the "get the power from the engine to the wheels" problem makes it work well in those use cases. Directly transmitting the energy without converting into electricity and back is more efficient until you start talking about absurd levels of gearbox and input/output speed discrepancies and whatnot. If the double conversion was more efficient we'd see electric drivetrains in far more places.