The signal room can only be opened when both the locks are opened together.
The room is a secure, heavily guarded room with no scope for outsiders to enter. And the relay can only be set from inside this room.
Besides, only someone with knowledge of the signalling system can make a change to the switch.
So sabotage is extremely difficult. And if an attempt is made to tamper with the wiring from the outside, the signal will turn red.
A signal isn’t a fish plate that can be removed.