I was doing a little of your mental exercise re: the alternating direction of each loop. An easier way to achieve it would be to add a second control DPDT relay that would reverse the polarity and point throw of the loop the train is currently on. The trick would be to lock the relay in position but not have it actually applied to the track until the sensor on the opposite loop or possably one located on the mainline is activated. Put one on each loop and they would alternate CW and CCW on each progressive trip. It would be a little more complicated, but not so much so that it wouldn't be be doable if you really wanted that feature.