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
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.