I like the bi-directional diode idea better than either of the ones that I came up with, actually, as far as dead simple goes. Just keep in mind you are going to need some really big diodes. The benefit of using the load bank would be that it could be made responsive to the change in voltage at the transformer to keep output at exactly 18V (or whatever) where as the diodes would drop in .6 volt steps, or in the simplest form a 2 volt or so jump.
As for how to read the transformer voltage, I would use a knock-off arduino micro-controller as the brains of the operation. I'll work out some details for you, if you like as I have time over the weekend, but the basic setup would be to rectify the voltage to DC, then use a voltage divider to bring the transformer's maximum power output down under 5 volts. this will give you a zero to 5 VDC voltage from your 0-24VAC transformer output. This signal voltage can then be read by the Arduino, which, in turn can be programed to turn on a relay at a set voltage to turn on the diodes, or turn on a varying, pulse-width modulated signal to a set of big power transistors on a load bank or triacs on the line to the track. If I went with the diode method, I think I would at least use a few relays to let the system have a few steps, clicking on more and more diodes in series as the voltage rises.
JGL
