As I understand, braking occurs across the entire train, not just via the drive axles, so the hybrid regeneration method would be a considerable departure from normal operations.
They're great for stopping (relatively) quickly on level ground. But not so grest for controlling your speed.
Big diesel-electric locomotives have large, actively cooled resistor banks, usually on top of the engine. When rolling down hill they run the electric motors as generators, and dumping the energy into the resistor banks.
The mechanical brakes can't absorb that much energy - they'd wear out after a few minutes.
It seams like a resistor on a generator would just be a breaking load, and not actually capture any power.