Well – that’s a pretty broad question – what sort of signal failure do you mean?
Generally speaking, a signal failure is any problem where a piece of information which is supposed to get from one place to another (that’s all a signal is – travelling information), fails to get there.
Like if a telephone line is cut, or someone un-plugs your broadband. There are loads of ways a signal failure can happen, as there are so many types of signal!
You mean signal failures like you hear on the train? I’m no expert on trains, but what I imagine is what they call a “signal failure” is a common term to talk about lots of different problems. Because the rail network is so connected, if one small thing goes wrong (maybe there’s a power outage in one station for just a second which causes some computers to have to be reset which takes time, or some sensor on the tracks somewhere gets iced over when it’s cold) it doesn’t only affect one train, but like a chain reaction to lots of trains and lots of lines, so you get delays and one tiny malfunction can cause lots of others.
I guess signal failures on trains can be caused by lots of reasons – but I know a pretty common reason at the moment is become someone has stolen the cable! The cables contain copper which can be sold for a small amount of money, so some people will go and risk their lives (and the lives of others) by stealing the signal cables that run alongside train tracks.