You may want to look into optocouplers/optoisolators.
All you need is a bi-directional optocoupler. I used LTV-844 optocouplers which have 4 channels per chip. It is called an optocoupler because the two leads that connect to the sensing side (insulated rail setup) illuminate a small LED. The illumination of that LED then closes a separate circuit comprised of a photosensor. Below is the break out of the LTV-844:
Pin 1 would go to the insulated rail, pin 2 to a fixed AC voltage source (or center rail) through a current limiting resistor. When pins 1 and 2 make up a closed circuit via metal axles, the LED is illuminated (for the astute, you will see that this selected optocoupler has two leds in reverse polarity so that they will work off of the AC voltage of the track). The light from the LEDs active the photosensor between pins 16 and 15, closing a digital circuit that can be connected to the low voltage logic of Chinese relay boards. When the block is occupied, pin 15 will read a digital 1 at VCC, and when not occupied, a digital 0 (again, pull down resistor recommended).
This also enables complete isolation of the AC track and the DC accessories so there is no concerns if you derive your DC from an isolated or unisolated transformer/power source.
