Modern electric passenger trains do - they can dump power back into the grid, and have enough motors to do a good chunk of braking with them (And e.g. for the G

Modern electric passenger trains do - they can dump power back into the grid, and have enough motors to do a good chunk of braking with them (And e.g. for the German high-speed trains they plan the braking points such that they try to avoid using lossy brakes in normal operation). If you don't have a grid as "storage", you need to carry it, and that's a problem. And freight trains obviously only have motors in the locomotive(s) and do the distributed braking on the cars mechanically, so a good chunk is lost even if the locomotive can do it.