Originally Posted by
Norseman
For the really heavy haulers like locomotives (and ships, etc), a big reason for being D/E hybrid is the simplification of the drivetrain. The huge prime mover ICE can only run in a narrow RPM band, which would be a challenge to mechanically couple to the wheels (impractically huge clutches, giant geartrains)... but by sending juice to the electric traction motors, there's no mechanical connection required b/w the diesel and the drive wheels. No drivetrain losses from inertia and friction, way simpler maintenance.