Amplifying the signal from a transponder detection loop
The clue is in the name - transponder. I suspect that the loop not only powers the normally unpowered transponder (fixed to the car) but also acts as receiver. At 5MHz the transmission process will largely be the magnetic part of the EM wave that is used.

It could be that the transponder (in the car) is normally powered but only transmits when it detects the presence of the magnetic field from the loop.

Either way, I suspect you need the trackside circuit board and (mag field generator and receiver) still to be trackside for this to work. Trying to convert it to an equivalent of a regular RF antenna is, possibly, missing the point. For it to operate as a beverage antenna it's length needs to be one wavelength and at 5MHz that is 60m long. It is a magnetic antenna not intended for EM waves.