Inspired by MP147's single non-latching relay method, here's a potential starting point that does not require insulated rails. Using ideas from rrman's thread on grade crossing detection, I cobbled together a circuit from spare parts and tried it on a simple Realtrax loop. Reed switches detect a magnet placed under the forward truck in each engine. Using a strong Nd magnet only 1/4" in diameter, I was able to place the reed switch under the track. Rrman describes how the heavy O-gauge steel wheels from a derailed truck slice through a reed switch casing placed top-side. The reed switch is off-centered on the left side of the approaching engine; likewise, the magnet is placed on the left side of the truck. This allows upgrading to bi-directional operation though that's a separate discusssion.
This requires electronic component assembly so I realize this does not meet the OP's criteria but the parts cost is under $5. To be clear, this does not have all the neat features Dale's method provides and I offer what I discovered in the spirit of a forum discussion.
A 555 chip is configured as a bi-stable latch so a cheaper non-latching relay can be used. The latch is set and reset by reed switches (one on siding, one on main). The set and reset switch signals are edge-triggered which means the stopping engine can sit over, or coast past, the switch once it triggers the changeover. I believe this edge-triggering feature allows easy adaption to ITADs. On the DPDT relay, one pole applies Track AC to the center-rail of the siding or main. The other pole configures both switches to straight or curve.
I only had a 5V DPDT (on the right) in the spare parts bin so the circuit runs on 5V DC provided by a 7805 regulator chip on the left of the board. To keep things simple everything including the switches is powered by track voltage.
The short video below shows two MTH PS2 engines locked in Forward. PS2 engines wait about 1-2 sec after power is applied to start moving. Considering this pause is “free”, it might relax the need for an additional timers. Also, PS2 engines have some built-in acceleration ramping in conventional so they do not jack-rabbit on start. Again, considering this ramping is “free”, it might relax the need for diode blocks (or whatever) to ramp up the speed. It also seems for engines with sound, it might be nice to lower (rather than remove) the voltage to keep sounds running while stopping the engine but that's yet another discussion.
