The 154c contactor has to sections on each side of the clip that goes over the outside rail. When a wheel runs over one half, it illuminates the left/right light, and when it touches the other it lights the left/right light. I dont think that the weight of the car has any effect on this. As long as it touches the outside rails, all it has to do is touch the contactor and make the 'short' as I call it and it illuminates the light.
I think Bob has a good point. The numbers may be different than the lionel flasher.