Higher voltages would create more sparks, which would deteriorate the rails and wheels quicker.
The current would be limited to a low value.
Online Sketch: Untitled Sketch by Anonymous Author Your drawing does not make sense. The motor should be across the tracks & the H bridge should control the direction.
First, rectification, then an inductor to prevent huge sparks when the wheel reconnects to the track after breaking, then big capacitor.
Then you need to control the motor's direction, this can be done with an integrated 4-mosfet H-bridge. Not represented is a sensing circuit which uses the last seen voltage on the rails to choose the motor direction.