I dont know whats being done to combat it, but lately horns seem to be the big thing. I know that there were horns stolen off of of a couple diesel-electrics sitting in the yard near my house, and I have been reading reports of this happeneing all over, since people seem to think they sound cool on their cars, but they dont realise how much air they require to operate.
Its pretty sad that people think they need to steal from the railroads. They need to be locked up for quite a while to learn their lesson.