Very rare. It seems like 90% of delays are due to "signalling problems" (which is probably also code for "we have no idea what is going on", but still...)
The signals are designed to fail safe, due to gravity. But because they are electromechanical they fail a lot more often than solid state digital tech.
Gravity doesn't play into it much any more; semaphore signals are increasingly rare by this point, certainly on any main line. Modern electric signals simply default to danger in the absence of contact to the signal box, and fail-safe behaviour is maintained by the rule book stating that a signal displaying no aspect is equivalent to a signal displaying danger.
Gravity is still very important. The way that electric signals fail safe is that the relay is mounted in such a way that if something fails then gravity pulls the contacts to the danger position.
Aren't relays usually lightly spring-loaded/tend towards the NC position irrespective of which way you mount them? Maybe railway signalling relays are different, but none of the relays I've ever seen leave that behaviour to gravity.
