Yup, missed that last change of direction, not going to work. It would need two direction switches.
I’m beginning to think a Wye is not going to work without some sort of relay or switch. Relays are cheap, with a DC coil and a diode to activate it only when the power pack is at one polarity, and DPDT contacts, it could provide the track polarity reversal without an extra DPDT toggle. If the OP wants it more automatic than driving the train to the bumper and flipping the reverse switch on the power pack, he’s out of luck. There has to be SOME intervention - or a whole lot of complicated beam break sensors on each leg to reverse the train when it clears each turnout, at the very least. Even in DCC, making a train reverse itself with no operator interaction requires a lot of extra ‘stuff’ besides the polarity reverser.
Even the loop with a spring switch and rectifier isn’t hands off - you still have to change the direction switch on the power pack in order to exit the loop. Unless you put in a sensor to operate a relay.
–Randy