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AN INTRODUCTION TO - page 26 / 72

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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

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