Track movement under freight train

[jglfan]

I'm not sure I've ever noticed this on a mainline, but it seems common on sidings. I've seen where at a rail joint, one rail will drop each time a wheelset passes over, while th adjacent rail moves much less. Loose fishplate perhaps? Anyway I thought it would be neat to model this. However, even when I used thinner ties on one side of a railjoint and omitted a railjoiner, only a loco was heavy enough to get the one rail to depress. I never attempted it, but here is a thought I had: Using a photo detector to activate a spring loaded device to pull down on a wire soldered to the underside of a rail. Each time a wheelset passes over the detector, the wire is pulled down, releasing immediately, the spring returning the rail. It would be pretty cool for a foreground siding that doesn't see too much traffic, just a setout or two.