Block signals only check the block directly behind in order to show "Stop!" or "Go!". Even in rather simple setups, this is not enough and leads to severe problems. The best example are station entries.
The limits of block signals
This picture shows examples: All platforms in the station are occupied. The two trains at the top are supposed to continue to the right, but the signals are red, as somewhere to the right (out of the picture) a train occupies the signal block. The trains at the lower two platforms should continue to the left. The two trains approaching the station and which now block the station entry approached the station when the platforms were already occupied.
The signals on the tracks leading towards the station showed "Go!", since the switches and junctions in front of the platforms was free, the other trains were already on the platforms. Between the switches and the platforms there are signals, so these are different signal blocks. Therefore the approaching trains were able to enter the signal blocks in front of the station, where they now wait in front of the platforms, as they cannot go on.
For the train to the left, this is not that much of a problem. At some point, the trains at the platforms will continue their journey to the right and the platforms will become available again. In the worst case, the train on the other platform leaves earlier and the waiting train is still stuck, even though the other platform would then be free.
The train on the junction at the bottom however blocks everything. It cannot continue to a platform, and the trains on the platforms cannot continue their journey either as they are unable to leave the station. The player has to intervene and manually resolve the blockage.
What is needed here is a signal which prevents the third train to enter the junction block when all platforms are blocked and the train cannot enter the station. Exactly for this reason presignals have been developed.
Here, presignals are used, so the signal to the left shows "Stop!" even though the block is actually free. Thus, it prevents the train from entering the junction and blocking everything. Instead, any approaching train will wait in front of the junction, allowing trains to cross the junction and leave the station. Then a platform is free and the waiting train can proceed.
The following sections discuss the complete functionality in more detail.
Presignal types
There are three types of presignals: Entry (yellow horizontal bar), exit (white vertical bar) and combo (yellow vertical bar), which combines entry and exit in one. All three types are useless on their own, they have to be used together to get the desired effects.
Presignals can be built in the same way as block signals. One can create two-way and one-way variants, for each both semaphores and light signals are available. From left to right here are entry presignal, exit presignal and combo presignals. The functionality does not differ between semaphores and light signals.
Passing
The rules for passing a presignal are the same as for block signals. A train can only pass a presignal when approaching it from the correct side and the signal shows "Go!". When approaching from the opposite side, the train will stop at the signal and turn back. Only when signals in both directions have been placed on a tile, passing in both directions is possible.
Behavior
Entry presignal (yellow horizontal bar)
This signal marks the begin of a presignal block and only shows "Go!", when there is at least one Exit presignal (white vertical bar) or combo presignal (yellow vertical bar) which shows "Go!" and the signal block itself is free.
Exit presignal (white vertical bar)
This signal marks the end of a presignal block and works exactly like the block signal. It shows "Stop!" when the block behind it is occupied, and "Go!" if the block is free.
Combo presignal (yellow vertical bar)
The combo signal is a mix of the other two. It acts as exit signal for the block before it and as an entry signal for the block behind.
The entry signal to the far left interacts with the combo presignal (left track) and the exit presignal (middle track). Both these signals show "Stop!", as the tracks are in use. This means all signals relevant to the entry presignal show "Stop!", which makes the entry signal also show "Stop!". The block signal to the right does not matter because it is no presignal.
What is interesting here is the different behavior of the combo presignal and the exit presignal on the middle track when looking at it from the other driving direction. The exit presignal just works like a block signal, the signal block (the junction) is free, so this side of the exit presignal shows "Go!". The combo presignal is different: It takes the state of all other combo and exit presignals into account. The only exit presignal is at the track in the middle and shows "Stop!", which makes the combo signal show stop also for the other driving direction. It does not matter that no train can actually get from the left track to the middle one!
In this example there are two exit presignals, the one on the right hand track showing "Stop!". The entry presignal shows "Go!", because the block is free and there is one exit or combo signal showing "Go!" (at the left track). If that signal to the left would show "Stop!" just like the one to right, the entry signal would switch to "Stop!". That is exactly the behavior which can be seen in the examples at the top of the page.
In the middle there is a combo presignal. This connects block 1 and block 2. For block 1, it is the exit, for block 2 it is the entry. The exit presignal of block 2 shows "Stop!". Since this exit signal is the only exit or combo signal of block 2, the combo signal between block 1 and 2 shows "Stop!" as well (since it acts as entry signal for block 2). Now, all exit or combo signals in block 1 show "Stop!", which causes the entry signal of block 1 to show "Stop!" as well. As soon as the exit presignal of block 2 switches to "Go!", the other signals switch as well.
If there is no exit or combo presignal which leads out of a block, all presignals act like block signals, including the one-way combo signal. The two-way combo presignal keeps showing "Stop!", regardless of any trains in the block.
Usage
By now these signals are only interesting for very specific tricks. Path signals are much simpler to understand and offer a more flexible approach. So, presignals have become almost obsolete.