Arguably one of the things which gives OpenTTD the possibilities it has, signals are essential to learn in order to construct anything worth mentioning. This page will attempt to you show you the basics of what signals do and how they behave.

Block signals

The best place to start is block signals. This is the very most basic signal which does only one thing - is green if the block in front of it is free, and is red if the block is occupied by a train.

Signal Block

Question is however, what all is a signal block. The answer would be, any place between two signals. This means literally any place, including "senseless" rail combinations.

Self-blocking

Wrapping rails around the signal, unifying the signal block, can cause deadlocks! This can be used in some special cases.

Signal directions

It matters which direction is the signal facing - it always detects the block in front of it.

When you build a two-way signal, it is the same as if you had two separate signals, each of them detecting a different block.

Pre-signals

Pre-signals share all the mechanisms how they work with basic block signals, but they can be red in some extra cases, dependent on the state of other pre-signals. There are three of them:

Entry signals

Detects Exit or Combo signals ahead. If All of them are Red, then this entry signal is Red too.

Entry signal is a stopper - a place where trains stop under some condition. This means that it is used in any spot where train chooses from multiple options (platforms, choice waiting bays, anything) and we need it to wait for at least one platform/thing to be free.

Exit signals

Exit signal is the end of pre-signal chain. It does not detect anything else than the block ahead of it.

Combo signal

Entry and Exit signal in one - detects Exit or other Combo signals ahead, and is detected by Entry or other Combo signals at the same time.

The signals which chain the pre-signals and thus can separate signal blocks, but keep the pre-signal chain.

Path Signals