[Piwoslaw]

In order to pump electricity back into the railroad grid, there have to be other trains that can accept that energy at that moment. In the Donner Summit example (or any other uphill/downhill) for every decending train there should be one climbing the hill. Yes, it's doable.

Unfortunately, not all electrified raidroads allow regenerative braking, the Polish rail system being an example. The owner of the cantenaries (overhead wires) doesn't invest in new technology which would allow it to accept energy from braking trains because the 40-50 year old rolling scrap metal doesn't have regenerative braking, while the owners of the trains don't pay extra for regen when buying new locos because the rail system can't handle it. Circle closes. Even the new subway in Warsaw didn't think about regen, and subways are the best place for it - there are always trains that are braking and trains that are accelerating.