7.3 Release of Route
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When the train has passed the home signal at station B and the signal is turned 'danger' and the release of the route is automatically started. The release current ("black") is sent from station B to station A.

When the release current reaches station A, it checks that the exit signal does not show 'proceed', i.e station A is not intending to dispatch another train toward station B. If it does show 'proceed', release of route will not take place. If signal A shows 'danger', a black current is sent back to station B.

 Route is released when the current reaches station B. The signal aspect currents ("yellow" and "green") are cut off and the block signal are extinguished. The "black" current from A is also cut off and the system returns to idle condition.


All the information is transmitted on a two wire under ground cable. The aspect currents "yellow" and "green" are d.c. with opposite polarity and the locking and releasing current, "black", is a.c. The wires are connected with filters in block posts. This enables simultaneous transmission of d.c. and a.c. on the same wire pair.

The locking relays in the block posts are of magnetic stick type and once picked up by the locking current, they remain picked up until the release current is applied. If a ABS post is locked and it receives no aspect current it shows 'danger' and sends a "yellow" current  to the post at the back. If it receives "yellow" or "green" aspect current it will show 'caution' or 'proceed' aspect and send a "green" current to the post at the back. When the releasing take place all aspects are extinguished. The controlled signal do not respond to aspect current but take it into consideration when a order to is given. It turns 'danger' as soon as the train passes it.

The spacing of block signals varies from 4000 to 8000 feet according to the curvatures on the running rail and the visibility. On double line area only one block is provided between station in the reverse direction.


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7.1 Route Establishment ] 7.2 Train on Line ] [ 7.3 Release of Route ]

R. Jayanthan
Last updated on 01 January, 2002