For three weeks YAPP (PBS) is in trunk now and we had some games using the feature of Path (Based) Signals. I had a closer look at this feature and some serious discussions with developers and other players. In this review, I want to collect and summarize the main aspects of YAPP as far as I understood them, show you new possibilities for terminus-like stations, pitfalls and much more.
Path Based Signaling
The history of Path Based Signaling started in early 2005 and the last try (YAPP) finally made it to the trunk in August 2008. Brianetta explained how PBS works in a pretty nice way:
Path based signals are technically new in OpenTTD, although they’ve been here before in another form, and TTDPatch has had them for a while. They act differently from block signals, in that they will allow more than one train to enter a signal block if they’re definitely not going to collide. On a straight bit of track, this means they act pretty much like block signals. If there’s a complicated patch of track (two ways in, two ways out, for example) and two trains won’t use the same bit of track, they’ll show clear and let them both through at the same time.
It should be very clear how we call the different signal types as it is very important for our cooperative/communicative games. Planetmaker came up with the hint that abbreviations like PBS and YAPP are words used by developers and players which are really into it. Therefor we suggest the following words for signals:
Regular Signals, Entry/Exit/Combo Signals are: Block Signals
PBS, YAPP are: Path Signals
To build path based signals you can either use the signal menu or cycle through the signal types by ‘ctrl + clicking‘ on the signal. In the patches -> constructions tab you can find several options for signal building:
- Enable the signal gui: On / Off
- Signal type to build by default: Normal / Advanced / One-way advanced
- Cycle through signal types: Normal only / Advanced Only / All
Path signals are really great, but in my opinion they also cause some problems which are important in our games. To become more specific I mean the Evil X Issue which is known since ages. Path signals do not solve this problem, but I saw a lot of those constructions since path signals are in trunk. Of course its fantastic to have more than one train in the same block, but path signals are as powerless as block signals are if trains have to cross. You should always keep this in mind. Path signals make a lot things much easier, but you should not start building carelessly.
Another issue is that I find it pretty hard to combine block signals and path signals. I was not able to construct any useful things combining both types.
In some cases stop in front of a path signal block even if there is a free track. I am not sure why it happens, though it could be a delay caused by the system to find/calculate the free track.
If a trains goes from A to D, another train going from B to C must stop. With a bridge or tunnel you can avoid this phenomenon: while one train goes from 1 to 4 another one can go from 2 to 3.
A very silly example I saw too often.
- More than one train can join a signal block if there are free paths
- Path signals can be used as penalty signals because reversed two-way path signals have a penalty. This is very useful because we don’t have to build penalty stations anymore (If I am correct its the patch setting yapf.rail_pbs_signal_back_penalty)
- In lots of cases you can save space when building splits with path signals (e.g. tunnels or bridges)
- The possibility to create completely new station and hub layouts
Terminus & 2-Way RoRo-Station Concepts
Of course we are interested in using the capabilities of path signals. The most considerable use of path signals is in the case of 2-Way RoRo Stations. 2-Way RoRo Stations, are roll-on-roll of stations which have entrance and exit on both sides. Principally they work like terminus stations having a terminus-like entrance/exit construction on each side. The problem in the era before path signals was that it is was impossible to find out which train wants to go to which platform and this could cause heavy problems and even deadlocks. Using path signals we can reserve a path including a platform and no other train can come into conflict. This behavior is not that important for terminus stations, though indispensable for 2-Way RoRo stations.
This is an easy six platform 2-Way RoRo Station setup, showing how the it works. I included several ideas. E.g. avoiding the evil X’s and replacing them with another clever construction, which uses a bit more space and makes the entrance/exit more complex but is necessary for a 100% throughput:
As you can see there is one evil X in the construction, but due to the reversed path signals these platforms have a penalty and trains join them only if all other platforms are already reserved. (I created this example in PSG#109 and its even included in the final savegame).
Planetmaker and I fiddled around with these construction and created two concepts for path signal terminus stations which can be easily extended to 2-way RoRo Stations by adding the same entrance/exit construction on the other side, too. The main focus was to create an entrance which is not wider than the station its self. It is one of the main disadvantages of my Terminus Station. Though there is one general rule: The narrower an entrance the longer it is and the wider the shorter it is.
Path signals are a great feature and have loads of potential. Nevertheless you have to care what you build because it could happen really fast that a construction is not effective as it should be with path signals. I have the feeling people think path signals are the solution to all the problems what is definitely not true. There are still (the old) unsolved problems. Keeping this in mind we will have much pleasure with the feature of path signals.