Typical permissive signals that can be found on lots of places in Belgium
Permissive signals in both directions
There are two basic line lay-outs for the bi-directional permissive signalling.
a) The oldest one: All permissive signals are released.
b) In the new lay-out all the signals of the wrong main running are lock until needed to clear for a train.
In the example below the line is in the normal state. Two trains are waiting to be signalled to the main line.
The exit route of the first train is released. The first train gets a green light without speed limitation.
When the train passes the main signal, the signal restores its default state.
The junction and the first block is cleared. Now the second train gets a free way. Speed in the junctions is restricted to 40 km/h.
The train approches the next junction. The main signals only open progressively although the runway can be already set.
When the second train passes the main signal, it restores its default state.