[B Knotts]

My understanding is that diesel-electric is not used in locomotives for efficiency; it’s simply impractical to develop a direct transmission of that size.

[Lonesome in Massachussets]

Probably true, which is another virtue of an electric drive, you don’t need a transmission. (Some homemade electric cars have transmissions, but that’s a vestige of their internal combustion origins.)

[Lonesome in Massachussets]

My understanding is that diesel-electric is not used in locomotives for efficiency; it’s simply impractical to develop a direct transmission of that size.

That still doesn't negate the efficiency of diesel vs. four stroke. The advantage of the four stroke is weight per HP (hence no diesel aircraft) and better acceleration. With a diesel hybrid, you could get the acceleration from the electric motor, long term power from the diesel.