A hybrid engine shines when you do a lot of stop and go, because the regenerative braking recovers the energy from the motion that would be wasted as heat in the regular vehicles with friction brakes.
Or regen will also help when you are going up/down hill, and recover the energy when you are cruising down slope.
But in a cross-country drive at a constant speed on flat land, there's not much an electric motor can do to help.
The electric motor can help with acceleration, and this allows a smaller ICE engine. However, trucks always have poor acceleration, and nobody seems to care to improve it. Truckers care more about low fuel usage than drag racing.
Also, a Diesel Electric locomotive also is not a hybrid, in the sense that we think of them as in cars. The way they work is that the Diesel engine provides power to the electric motors that turn the wheels. It eliminates the need for a transmission, which in a locomotive with 8 or more wheels, would be very complex, and wasteful.
I might be over-simplifying it a bit, but, but they're not like an automotive hybrid where you mostly run on the electric, and the gas engine only kicks on when you need the extra power, or to recharge the batteries.
As for hybrid cars, while they usually get better fuel economy in the city cycle than the highway cycle, for EPA purposes, don't they still usually get better economy, overall, than their full-gasoline counterparts? At least, when comparing across similar size classes? For example, I believe the Prius always got better economy than a Corolla...by a big margin on the city cycle, but even on the highway cycle I think it was better.
So, perhaps a hybrid system would still do some good in a big rig, if not as much as we'd hope for?
