The Z-1000 brick (not the controller) has a fixed 14V AC output on screw-terminals. Is/was it inconvenient/impractical to run an extra wire from the brick to the switch? Same question for power the tunnel lights.
If you can run the extra wire from 14V AC to the tunnel, you can use ~$1 of diodes to lower the 14V AC from the brick to ~12V AC. But if you absolutely, positively, want to power the 12V AC bulbs from the variable AC track voltage then it comes under the category of anything is possible given enough time and money!
So what are these three 12V AC tunnel lights you have? Bulbs can typically be powered by AC or DC. If DC can be used, additional options are available.