From #openttdcoop wiki
A Presignal Guide
(under construction) A guide for newbies, about presignals. However, this is not only for beginners as there are many complex things concerning presignals. If you know how the presignals work, you should skip the "basics" Part.
Part 1: Basics
The Presignal Basics is explained here: how they work, how they behave, and how does a terminal station work, and what's the use of the combo presignal.
Part 2: Oneway or twoway Presignals
Signals have two sides. In the basics part, the other side wasn`t important, since trains can`t turn back, etc. However, in more complex situations, a badly placed two way Combo Presignal can cause errors in your network.
Part 3: Priorities
When you have to connect two tracks into one, the most used track should have more priority than the lesser-used track. In this Part, junctions with priority are shown.
Part 4: Load Balancers
How to make presignals on a Load Balancer.