Node:Pre-signals (detailed), Next:New Graphics, Previous:Sign Cheats (detailed), Up:Additional Reading
Usually, making an efficient station where all trains can go to all platforms is very difficult, because if all platforms are in use, the trains randomly choose one and wait till the train occupying it leaves. If that train is set to "full load" this may take a long time, the station will be blocked until that train leaves. One workaround is to have "overflow" tracks, where the train goes if everything is full. While this works, it makes the train run all the time while waiting for a platform. This is very inefficient, and it takes up a lot of space which might be an issue especially for a city station.
These new pre-signals on the other hand work similar to what is also often called "home signals" or "entrance signals". The regular TTD signals are called "block signals", because they always indicate whether the block protected by them is occupied, no matter where the train actually wants to go.