Train engineer here; most modern diesel-electric locomotives are series hybrids, with the Diesel engine acting as a generator powering electric motors which turn the wheels. They are not (in most cases) powered by the battery. There are batteries onboard to provide power for things like lights, air conditioning, radio, computers, etc. but they’re relatively small and either can’t power the wheels or couldn’t get very far if they could.

There have been some experimental battery electric and battery powered units with range extenders, but nothing that’s really gotten any mainstream use on the main line.

Dynamic braking is achieved by using the electric motors as generators and sending the electricity to the dynamic brake grid, which is essentially a large toaster oven coil with a fan. The electricity generated by the dynamic braking is used to heat up the coil and run the fan, which blasts the heat generated out a vent in the side of the locomotive. Often when you hear of a fire in a locomotive it’ll be the dynamic brake grids blowing out.

The point is that currently all that energy generated by dynamic braking is just converted to heat and blasted out into the atmosphere. Battery size has nothing to do with dynamic braking capacity or efficiency. The exception is electric locomotives which will collect that energy and send it back into the catenary to be used later. Maybe one day when battery electric locomotives are a reality on the mainline dynamic braking will be useful for more than creating heat. Or we can convert all the lines to electric catenary.