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 are 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 - removed type of 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. PBS is dropped out of from nightlies due of its annoying bugs.
Part 2: The Station Problem
Stations without any presignals have a big problem: just have a look on the Image to the below. Signal A should be red...
Problem at stations without Presignals
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.
How does it work? If Exit Presignals B, C are both red, then Entry Presignal A is red too. Then a third train cant pass trough Signal A.
Part 3: An Effective RoRo station
Combo Presignals are mostly placed between Entry, and Exit presignals, because the Combo Presignals transmits the Exit presignal signs fot the Entry Presignals.
In the Image above, signal A is Entry Presignal, signals B, C, D, E are Combo ones, and F-K signals are Exit Presignals.
If F and G Platforms are used, then Combo signal B uses his Entry Presignal alibity, and it will turn to red, because F, and G Exit Presignals are both red. But the Entry Presignal A is remains green. Exit presignals H-K are green, then Combo Presignals D, E is green, then Signal C is green too. With this Combo Presignal B is red and C is green, then the Entry Presignal A is remains green.
The Entry Presignal A will be red only, if all Platforms are used, Exit Presignals F-K are red. If F-K are red, then all Combo Presignals B-E are red too, then the Entry A will be red.