I assume the issue is trains that aren't electric, and I assume even most electric trains don't have significant energy storage on the train itself, so might not really have much to do with energy from breaking.

It probably especially makes sense that freight trains running off fossil fuels won't even consume much electricity at all (just enough to operate instruments in the locamotive?), though presumably passenger trains could make use of it.

Maybe there are barriers to electrification of some trains in some areas.