How do the pre-signals work in OpenTTD? [duplicate]

Question

I'm reading their Signals article on their wiki, and it's still not 100% clear to me.

I'm trying to (and failing) to understand the Entry signal, (and subsequently, the Exit and Combo signals, because Exit "Works the same as Entry but..." and Combo "Combines Entry and Exit").

So here's what it says on Entry signals:

Block Entry Pre-signal

An entry pre-signal prevents trains from entering the following block, unless there is at least one green exit signal on the opposite side of the block. This prevents trains from entering the block until there is an available exit and obstructing it. Note that depots have a built-in two way signal (it works as an entry pre-signal in a pre-signal set). If there are no signals designated as exits on the block directly behind the entry pre-signal, it behaves as a normal block signal. This is convenient for bi-directional pre-signals where only one direction needs pre-signal functionality.

Can anyone please provide a clearer explanation, and perhaps tell me how do Exit and Combo relate to it?

Answer 1

The best way to illustrate this would be to use an example. Say you have a station with two platforms, and two sets of signals A and B.

If you use normal block signal here, and both platforms are occupied, the signal at A will be green, allowing the next train that comes along to move into the track between A and B. This is bad because if the train from the other platform were to finish and move off first, the train will still be stuck behind the other train, unable to move. This is clearly inefficient, which is where pre-signals come in.

By placing an entry pre-signal at point A, and exit pre-signals at the two tracks at B, you can tell the trains to enter A only if the track after any one of the exit pre-signals at B is clear, thus resolving the situation mentioned earlier.

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A combo pre-signal is basically a combination of the two. It's useful if you have a large station with many more branches, like the one shown in the wiki:

In this case, the next train will move up to the topmost platform, and the first entry pre-signal will turn red, preventing any more trains from entering the tracks between the exit and entry pre-signals until at least one of the four have moved off.