I think the idea is that if you're driving a tall vehicle and you hit the bells, that tells you you'd hit the overhead power lines the trains use if you were to proceed. I could be wrong, though, as I've never seen these anywhere else (or maybe I have but didn't notice them).
I don't suppose the sound of a few bells ringing would stop a driver from proceeding to take out the power for the Colchester to Clacton branch line, given that only yesterday a bus driver managed to hit a bridge in Chelmsford that even I knew was low, but they're a quaint feature anyway.
I wonder if they ring in strong winds and annoy the locals. I think the clappers are still in them.