Mook, with an all AC system that is on the electrical grid, a downhill train would be able to pump power back into the grid through the transformers that take line current and turn it into current for the railroad catenary.

The light rail system, being DC, can't really do this. Its substations have a transformer and a rectifier (and other components), a large scale version of the power pack for an HO train. Without a load in the light rail electrical block, the power from regenerative braking DOES have nowhere to go because it can't go backwards through the rectifier to the transformer and back into the AC power grid.

I'm not sure how that worked with motor-generator sets like the Milwaukee and other older DC operations used. If the train pumped DC back into the wire, it could turn the DC side of the M-G set as a motor, but if the AC side was not spinning at the right RPM to generate a 60 Hz current, you couldn't put a load on the AC side by connecting it to the grid.

Make sense?