Track Circuit – Block Occupied
When a train enters the block, the leading wheel set short circuits the current, which causes the relay to de-energies and drop the contact so that the signal lamp supply circuit now activates the red signal lamp. The system is "fail-
safe", or "vital" as it is sometimes called, because any break in the circuit will cause a danger signal to be displayed.
Automatic block system using 3-aspect signals
This diagram shows a line with 3-aspect signals. The block occupied by Train 1 is protected by the red signal at the entrance to the block. The block