Conventional freight wagons are unpowered yes, but I don't see how that's relevant to this claim that somehow if you have multiple electric motors you can't dump power into the traction feed when decelerating. British passenger trains do this today, it's not even a quirk, it's the default. The local train operator boasts that they dump 78,700,000 kWh per year from deceleration back into the traction feed. [No I don't know why they picked those units]