Many actually have big radiators to dissipate energy regenerated while going downhill by their electric traction motors.
Hopefully, as batteries get less expensive this changes.
https://www.popularmechanics.com/science/energy/a39372219/se...
Here is one (I hope) that isn't paywalled: https://reneweconomy.com.au/fortescue-starts-work-on-world-f... Failing that try googling "infinity train battery". It was a popular story at the time. As the story says, at least two companies are doing it in Australia.
The amount of destruction these things can cause when they lose control of all that energy is impressive: https://www.atsb.gov.au/publications/investigation_reports/2... and https://www.atsb.gov.au/publications/investigation_reports/2... They can not be stopped in a controlled fashion once they get going. The latter one cost BHP $800M from memory.
Serial hybrid seems like a great solution for trucking, since you don't need to oversize the engine in order to cover high torque needs (the electric motors would handle that), and during flat highway drives the generator could top off the battery using a narrower RPM range.
Long distance trucking is also a place where I think green hydrogen fed fuel cell trucks could work, since range would a non-issue, and also the hinterlands, where long distance trucks go, is where most of the renewables are.
If the renewables can be fed to an electrolyzer nearby, which supplies a hydrogen fueling station nearby, then you don't need to build much transmission/transportation to get the hydrogen to the trucks.
This requires electrolyzer cost to drop a lot (and FC Semis to be available), but all the technology components of the system exist in some form.