Someone mentioned relays. I once solved this problem the hard way with a circuit board in each car that worked on track power, and created 5v regulated DC that powered a 5v radio shack relay. The logic was: one pickup was the default, and if it saw power, the car lamps lighted, and the relay remained inactive. If the roller saw NO power, the relay would trip and default to the other pickup. If it saw power there, it tripped so fast, you saw no flicker as the relay switched over. Better still, no current passed from one roller to the other as they were now isolated. After making enough boards for an eight car train, however, I was pleased with the results, yet I questioned whether it was worth the labor to do another train. But it can be done.
Well, that's certainly the hard way, but interesting. 
