Path signals are the standard signals in OpenTTD/JGR's patch pack. By default, they are also the only signals that are shown in the signal building menu, all other signals have to be activated explicitly in the options. The development of these signals was influenced by lessons learned from the path-based signals in TTDPatch. Features like marking a reserved path were taken over. On the other hand, many of the old signal features were simply thrown away because they were not suitable for the new concept. In the end, the new system is a very simple one, yet at the same time it is extremely powerful and quite realistic.
The image shows three trains moving within the same signal block next to a station at the same time. Each train has reserved a path which is marked in darker color.
The idea behind the path signals is comparably simple: Instead of just dividing the rail network into signal blocks, the blocks themselves are now divisible into paths. A path is the actual railway connection that a train will use to cross the signal block. Before entering a signal block, a train will try to reserve a path through the signal block. If it can do that, it can enter the signal block, whether or not other trains may have reserved paths within the block. Paths cannot touch or cross, therefore trains cannot collide with each other. If a train cannot reserve a path, it will stop at the signal.
The following sections will provide some more detail on the signals.
Path signals can only be built in one per direction per tile. The two tracks to the left show the standard path signal, both as semaphore and light signal. The other two tracks show the one-way path signal. The semaphores and light signals only differ in the looks, not in behavior.
Passing
Here there is a significant difference to all other signals: Standard path signals (the two tracks to the left) are ignored by trains when they approach them from the back. That means trains can pass them in both directions.
The one-way path signal (the tracks to the right, the signals with the red-white marking) can only be passed in one direction, same as the block signals.
This seems counterintuitive, but is realistic. It is also one lesson learned from the implementation of path-based signals in TTDPatch, which ran into limitations that could only be solved by adding additional signal types. In OpenTTD these problems have been inherently solved. As a player, one only needs to know two signal types and can start building complex networks.
Behavior
Path signals always show "Stop!", until a train has reserved a train past it into the block behind. As soon as the path is reserved, the signal switches to "Go!", only to switch back to "Stop!" once the train has actually passed the signal and entered the signal block. This mimics the behavior in reality, where signals are also always set to "Stop!" for safety reasons, unless they have been explicitly switched to "Go!" for a specific train.
There are standard path signals in front of this junction. The train on the track in the middle has reserved a path, the corresponding signal shows "Go!". On the right hand track, another train approaches a signal. It has not yet reserved a path, hence the signal still shows "Stop!". Once the train has reserved a path, the signal will switch to "Go!".
Here we have standard path signals on both sides of the signal block, always facing the junction. The destination of the trains is somewhere to the right behind the junction. As can be seen, trains ignore the signals when approaching them from the wrong direction, reserving paths across these signals. Keep this in mind when placing path signals!
Now, the signal to the right is a one-way path signal (red bar). It cannot be passed from the back side. So, the train has reserved a path on the other track, the second train has to wait until it can reserve its path.
Usage
Path signals are the standard signals in OpenTTD/JGR's patch pack. They are well suited to just about any kind of signalling task one may encounter in the game.