From #openttdcoop wiki
The Presignal Basics is explained here: how they work, how they behave, and how does work a terminal station, and whats the use of the combo presignal.
Part 1: Identify the signals
There is five type of signals in OTTD. If you click on a normal signal with CTRL is held, these signals will appear:
-Normal Signal: It doens`t have any special alibity, it turns to red if the next signal block is blocked by any train.
-Entry Presignal: It works like a normal signal, plus it turns to red if all exit presignals, or combo presignals are red after it.
-Exit Presignal: It works same as normal signal, but it signs for the entry, or combo signals, if self is red.
-Combo Presignal: It have Entry and Exit presignal alibities both: It turns to red if all Exit/Combo are red after it, and it signs for the Entry/Combo Presignals, if the self is red.
-PBS Signal: Not a Presignal, it enables more trains in one signal block, since PBS means Path Based Signaling. This type of signal isnt explained here.
You can see how they looks like here.
Part 2: The Station Problem
Stations without any presignals have a big problem: just have a look on this Terminal Station. If you let one more train to enter into the station it will block the exit, because A signal is green. This problem is solvable with presignals: Just turn A into Entry presignal, and turn A and B signals to exit signals. Like in this picture. How it works? If Exit Presignals B, C are both red, then Entry Presignal A is red too. Then a third train cant pass trough Signal A.