Late to the party but to add to what @andis said regarding fail safe system (break in rails, flooding).
It is fairly common for me driving light DMU's (14x) in leaf fall season for the train to disappear off the track circuit due to poor contact between the axle and rails (leaves coating the rails). Not only is it like trying to stop an 80 tonne car on ice using a handbrake but the signal behind you can clear because you are not activating the track circuit. Control signals can be held at red but automatics will clear to the least restrictive aspect they can show.
In this situation a leaf fall protocol is put in place and signallers are watching for trains disappearing off the track circuits. The leaf fall protocol is basically one train working. So between two controlled signals only one train is allowed in. The automatic signals then can do what ever they do without the risk of two trains being in the same section at the same time. Once the train has got past the second controlled signal the signaller can allow another train past the 1st controlled signal. Makes for slow progress as control signals can be quite far apart if the line has no junctions or points to protect.