A WP signal maintainer once told me of a signal which stayed green when it was supposed to be red. This occurred because someone had shot at it, and the bullet lodged in the vane mechanism, jamming it. This wasn't a mechanism failure either.

They're being replaced because they are relays and as such subject to the CRF mandated periodic testing procedures. They're also on top of the mast, which requires someone to climb up there to do this. Colorlight signals aren't relays, so the only time you have to climb up there is to replace a lightbulb. When these are changed to colorlights, a relay is added in the signal case, where it's easy to test.

What does "intermediate" have to do with the price of eggs?