A PTC is basically a slow-blow fuse you don't have to replace.
PTC's increase resistance as temperature increases due to increased current flow. Designed to limit unsafe currents while allowing constant safe current levels, resistance will "reset" automatically when the fault is removed and temperature returns to a safe levels.
The reason I use them is if you derail either the engine or the tender and the center roller sits on the outside track, full transformer current can flow in the tether wiring. Unless it's pretty robust, what usually happens is it cooks the tether necessitating a repair of the tether. The PTC just protects against that eventuality while not affecting normal operation.
