In the figure below consider the line between station A and Station B, which
is divided in to 4 blocks. The signals at A and B are home signals (controlled
signals) and the signals 1, 2 and 3 are automatic blocking signals. In the
reverse direction the entire track segment is treated as one block.
Establishment of route
In the idle condition controlled signals show 'danger' and the ABS signals
When a train is to be dispatched from station A to station B, a "black"
current is sent from station A to station B.
If the line is free, i.e. all track relays are picked up, the "black" current
arrives at station B and blocks there the dispatch of a train from station B to
station A. Station B acknowledges by sending a "yellow" current. The
first block post (signal 3) signal lights up and show 'caution'. This block
post send the current on, but now as "green" current.
The "green" current passes to the remaining block posts and their signal
switch on to 'proceed'. When the current reaches station A, the home signal
(starter) shows 'proceed'. There after the black current is cut off. The
direction of movement is now locked.
[ Continued... ]