Another discussion lead to trains and that got me thinking (yes, it happens
):
When does a train need the most power? Either going up a hill or starting from a station/junction.
When does it have a surplus of energy? Either going down a hill or braking before a station/junction.
So, the electric assist would not only be on hills, but also within 2-3 miles of a station/junction. On a hill, electric helper engines would be added to the train, but in the case of simple acceleration/deceleration the loco itself would have to be able to channel the energy into the catenary. How much would have to be changed in the construction of a typical diesel-electric loco to allow it power its wheels straight from the overhead grid? Since the catenary cables wouldn't be longer than 2-3 miles in each direction, then there is no need for outrageously high voltage, the voltage can be what ever is used in the electrical side of the engine, so no need for the large, heavy transformers that make up most of a pure electric loco.
And what about storing energy on-site? Instead of timing trains to go up and down hills, or start and stop at a station, at the same time, the hill/station/juction could have a device for storing energy. Since it wouldn't be mobile it could be large and heavy, and since it would be in an area with frequent train passes it would only need to store energy for no more than a few hours, usually for less than an hour. I'm thinking water pumping, flywheels, ultracapacitors, whatever. This would also allow the device to be gradually charged from the grid and then give that energy back in a burst.