Some are electic drive with the diesel as a generator. Regen braking simply pumps the power through a giant resistor grid. So unless you can employ a trolley assist system (similar to a tram- electricity from the grid, power fed back into the grid under regen braking), the benefits from mechanical drive are greater.
One some of my slow days whilst truck driving i noticed the following whilst playing with the instrumentation:
- air-con on uses 5L/ hour of diesel when idling
- Idling uses ~35 litres per hour
- Braking uses 4L/ hour
- Machines that can work 24/7 365 days a year in 50+ deg C heat and haul massive loads are awesome machines
- using 95% engine load as opposed to 100% load saved ~50L/100 when travelling up hill (for only a 0.5 km/hr drop in speed)
It all could add up. especially if you have a fleet of over 50 trucks. however most people go straight for the silver bullet options (biodiesel, diesel/LPG) and then wonder why it takes so much effort to work (and often falls over in the process)