Remove the bottom cover and check the operation of the(black) micro-switches at either end of the switch mechanism's travel.I had the same problem, the switch mechanism was not closing the micro switch completely on one side and the train wheels were activating the non -derail feature, causing sparks because the switch did not "know" the it was already thrown in the correct direction with the micro-switch not being activated.
To test the micro switches, manually throw the switch in either direction with the lantern,at the end of travel in each direction watch the micro-switch and listen for the "click" telling you its closing correctly, if theres no "click" its not correctly completing the circuit.
I fixed this problem by simply adding a small piece of tape to the side of the switch mechanism where it contacts the micro switch, this enabled the switch mechanism to close the micro-switch fully by acting as a bit of a shim. The switch has never been a problem since.
Additionally I and a few others have had a fastrack switch "jamm". I found that the switch motor was not snugged completely allowing the gear to ride up out of the rack, jamming it at the end of the switch mechansim, tightened the motor screws, problem solved.
Hope my directions aren't too confusing, they will make more since when looking at the bottom of the switch with the cover off. Its always easier to exchange something if you can, but for me the worst part of opening these switches up is removing the dozen or so small phillips screws from the bottom, the mechanical part is fairly simple.