The guard, driver & signalman are there in an emergency & they are trained to a very high standard. The drivers have to learn to be guards first so you have 2 trained guards on your train & They have to learn firefighting, advanced first aid & a whole host of safety things, they are like a cross between a fireman a paramedic & a policeman essentially.
The way the signalling on the railway works these days is called 'Track circuit block' It is a fail safe method & it ensures the signalman know exactly where every train is on the network at all time if anything happens 'they know instantly' The guard & driver know every inch of the track they travel & are tested stringently they have to literally recite every inch of it, Just Clapham Junction alone would take some serious study, I believe a train passes through Clapham every 6 seconds.
Since it has been privatized in the UK now I don't think it's what it was in terms of safety, they are talking about doing away with the guard all together now in my own opinion they are making a huge mistake & will probably end up having to revert the whole thing back 25-30 years I hope they don't, I can just see a huge public outcry after impending disasters that are sure to happen as they continue to abandon the old 'safety first' method of the nationalized system. To come in line with the 'protect profit at all cost' business model.