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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 six types of signals in OTTD. If you click on a normal signal with CTRL is held, these signals will appear:
Normal Signal: It does not 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 has both Entry and Exit presignal alibities: 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 is not explained here.
One-Way PBS Signal : It works like a PBS Signal, however, it prohibits passing through from the back side. This type of signal is not explained here.