Diesel electric drive are only around 80% efficient while a mechanical transmission is 95%+. It is a misconception that trains use them for efficiency, trains get their efficiency from low rolling resistance from steel wheels on steel tracks and aero efficiency from all the cars drafting together. The electric drive is needed for simplicity of routing power to wheels and for the precise traction control needed to get the train moving without slipping the wheels.

Yes, diesel engines are some of the most efficient internal combustion engines, the best of them are slightly better than 50% efficient from tank to crank.

And then diesel-electrics have more losses converting that to electricity, and then more losses again to convert back to rotational power.

It is more efficient to generate electricity at a power plant and electrify the tracks, because power plants can far exceed 50% efficiency. Also they can run on something less dirty.

That’s mostly because of the difficulty of making a mechanical transmission that can survive the required torque. That used to be impossible, now it’s merely a bad idea.