There is also the Azatrax MRD1-V Detector operating in the one shot mode. Set it to max time 14 seconds.
Have an isolated track where you want the trolley to stop just before the bumper. Have a short live track that mounts the bumper. The live track is where you mount the detectors. Operation: The trolley hits the bumper and trips the detector. The trolley reverses and enter the isolated track which now has no power for 14 seconds. When the detector times out, the already reversed trolley proceeds.