Many electric transit systems use regenerative braking. However, as other noted, if there's no load to accept the generated power it's worthless, and has to be dissipated on the car as normal dynamics. One alternative is to add storage (batteries, ultracaps) to substations for, say grades or flyover bridges. That can capture power from trains running downgrade and supply it to trains running upgrade later, reducing the power bill. Cost-effectiveness is still marginal because of the cost of batteries and ultracaps, and the control systems.

Essentially, we're talking about net metering, which is a thing in residential solar and wind systems. You generate x kwh. If you use more than that at any given time, it just reduces your use of grid power; if you use less, it feeds back into the grid "running your meter backward." In Europe, it often is "bought back" by the utility at a very low wholesale rate rather than directly offsetting the bill.