In the beginning the lines were only protected with station entry signals. Only one train was allowed between two station. These lines still exist in Belgium, but with 2 main lines with 2 tracks. For example the entry signals at Luttre. Here four tracks (2 lines with 2 tracks) arrive from Charleroi. If one track is obstructed, a train can't be signaled wrong track but is signaled on the other main line.
Signals in Luttre. Only track 1 and 3 have an entry signal
Wrong main entry signals were implemented. This configuration still exist on tracks with low volume.
Because the increase of traffic, permissive signals were inserted on the left-hand track. Multiple trains can run in one direction. But only one train can run on the right-hand track. In this configuration still a lot of lines occur. But in the time they will be upgraded to configuration 5.
To increase also the volume of trains signaled in the wrong direction, extra permissive signals were inserted.
Nowadays they prefer to keep the signal closed in the wrong direction. They open all on the same time when needed to signal a train.