Hybrids main use are stop and go urban vehicles, on the highway at constant speed nothing beats a diesel. In gasoline vehicles you can go Atkinson cycle so that the gas engine approaches diesel efficiency with a loss of torque which is made up with electric motor. However if you're driving long distance all time might as well just go diesel like all other long haul vehicles.
Math does not work out, series hybrid will be around 80% efficient while mechanical direct drive is 95%+. This is why nearly all hybrids are parallel and at highway speeds directly connect engine to wheels.
Trains do diesel electric for precise traction control not efficiency, they make up the efficiency loss with low rolling resistance and aero drag.
