Theory

The signal menu opens as you select the signal icon from the tool bar. The signal type can be selected from this window, with the upper row showing the semaphore versions, the lower row showing the light signals. Both semaphores and light signals act identically.

Terms and descriptions

To understand the theories in the following paragraphs, it is necessary to know some terms:

signal block

A signal block consists of all pieces of track between entry signals and exit signals. There can be up to 255 signals for a signal block, which should be more than enough. There can also be blocks without entry/exit signals, so that trains cannot enter/leave the block, but these setups do not any have practical relevance.

entry/exit signal

Entry signals are all signals that lead into a signal block, exit signals lead out of the block.

All signals are entry signals and exit signals at the same time (exit from one block means entry into another block). The actual type of the signal is unimportant in that respect (it is important for efficient signalling, though)!

Just in case you did not understand what I wrote just above, I'll try to give an example: Imagine a huge labyrinth of chambers. All chambers are connected by doors. In other words: Each chamber is a signal block, each door is a signal. Any door through which one can enter a chamber is equivalent to an entry signal, at the same time one can leave the other chamber through said door, which is the equivalent to an exit signal. Doors may be closed at any time, usually because somebody else is in the chamber behind that door and only one person per chamber is allowed. A closed door in this sense is the same as a signal showing "Stop!". Quite easy to understand, actually.

Examples

The signales here are only placed to show where the signal blocks begin and end! These examples have nothing to do with correct signalling!

This is a very simple signal block. One signal leads into it, the other one leads out. Remember that the area before the first signal also acts as a signal block, as does the track behind the second signal.

Here, one track branches into three. This is the common case at junctions, where signalling becomes important.

The inverse is also possible, four tracks merge into one here. In such a case signals are vital for directing traffic.

A signal block does not have to consist of just railway tracks. Here the station forms one signal block, guarded by a two-way signal to allow entry and exit.

Behaviour

In TTDX, a signal can either show "Stop!" or "Go!", depending on a number of conditions. This is true for all signal types, also for pre-signals and path-based signalling! Both enhancements only change the conditions that determine whether a signal should switch between the two states. A light signal shows its state by a green or red light. Semaphores raise or lower their arms.

In these guides I use the original signal graphics. There are other sets with signal graphics, which are based on real-world examples. These show "Stop!" and "Go!" in other ways.

Example for semaphores and light signals, each showing "Go!" (left) and "Stop!" (right).

A train will never, under no circumstances cross a red signal (the player may enforce it manually, though). It will either try to find another way, or it will stop at the signal, waiting for it to turn green. A train will also never cross a one-way signal from the wrong side (except for the so-called through signals). If a signal has not turned green after a certain amount of time, a train waiting there will turn around and try to find another way. The waiting time can be set.

Pathfinding

Before discussing the influence of signals on the behaviour of trains it is important to take a short look at their pathfinding. Think of a moving train as the result of two processes: pathfinding tells the train where to go, and the signals tell the train when to go. Usually, both processes are connected in various ways, i.e. as soon as you build junctions and signals a train will not move without the results of both processes.

OpenTTD uses a completely rewritten algorithm called YAPF (yet another path finder). This algorithm can look for paths through the network much more cleverly than the original one, so the limitations from the original engine do not matter anymore. In addition, the new system is much more flexible.

As players, you only need to know that the pathfinding also takes signals into account when calculating the best route. Each possible path will be given a certain value, depending (upon other criteria) on the number of signals which show "Stop!". The path with the best value is then chosen.

One can influence the weighing factors of the pathfinder in the configuration file openttd.cfg. However, you should really know what you are doing there and how this could affect your game - it is a much better idea to just rebuild the problematic points in the network, if something does not work as expected.