Pre-signals function as block signals, but additionally decide on which colour to show by the status of other pre-signals in the same direction down the track. Specifically, an entry pre-signal allows trains to enter if, and only if, there is at least one green exit out of the adjacent/following block.
Signals are used to direct your trains and prevent crashes. This tutorial provides a basic introduction to signalling. For in depth information on the different signal types and how to use them, see Signals.
Pre-signals and a single track intersection This example should give you a good idea of the help that pre-signals can be, and an example of combo signals. For example, a line has two trains, each going to its own station.Both trains travel along a single north-south track for part of the way. You'd use regular two-way signals where the lines switch onto the single section (four in total, one ...
The use of one-way PBS signals dedicates a platform to one specific direction, which decreases the chance of a deadlock. (Most of the times a deadlock occurs when all platforms are occupied by trains wanting to travel in the same direction, but the way out is clogged) Notice the use of one-way signals.
Two Tracks When using a single track for multiple trains between two stations, a train may need to wait for a long time before it can use the track, especially when the track is long. The reason is that only one train is allowed on a single block of tracks (the section of track between two signals) to prevent train collisions. To solve this, you need double tracks so that trains can pass each ...