I was hoping for a reply to this as it's something that I'm interested in trying but I'm useless with lectrics.
The way I thought of doing it is setting the trains so that they run in opposite directions fir the same controller input, reed switch under each track in the passing loop that will only be fired when the whole train is in the loop: train comes in, reed switch is 'flicked' which in turn fires both point motors via a capacitor, thus the power is cut to the incoming train and sent to the other one which then leaves in the opposite direction. When second train comes in it 'flicks' its own reed which fires the two point motors the opposite way to let the first train out again. Thus the power from the controller is live and in the same direction all of the time.
Whether it would work or not - I've no idea because, as I said, I'm useless with lectrics.
I did do some thing similar a while back but rather than reed switches I used two bits of phosphor bronze stuck up out of the track. A tab on the train hit the phosphor bronze, making a contact between the two bits which fired the point motors, and the momentum of the train took it past the phosphor bronze bits before it stopped, thus they acted as passing contact switches.
But I should re-iterate, I'm useless with lectrics and I give my account her not as an answer that anyone else should try (don't blame me if you set fire to your layout) but just to try to kick start the thread