As soon as the train passes the controlled signal at station A, the signal
switches to 'danger'. Note that the "green" current is short circuited
by the train axle and does not reach signal A.
Sooner or later the the controlled (home) signal at station B will be set to
'proceed' by the controller. As a result signal 3 switches to 'proceed' as well.
When the train leaves the first block, the the signal 1 switches to
'danger' since the "green" current is now cut off by the train. This
signal post sends a "yellow" current backward. Since the signal A is a
controlled signal it ignores the "yellow" current and remains 'danger'.
As the train enters block 3, the "yellow" current set the signal 1
to 'caution'. "Green" current is sent to the previous signal. Now a
new train can be dispatched on the line from A to B and ABS ensures at least on
block space between the two trains.
The train proceeds and the "green" current switches the signal 1 to
'proceed'. Now you can deduce the aspect meanings of the ABS signal.
||Stop! Train on next block
||Train on block after next; proceed with caution
||Normal speed; next two blocks are unoccupied
[ Continue... ]