A terminal station is a station where a line ends. All trains enter and leave the station into the same direction. These setups are usually common for city stations, where space is scarce and junctions cannot be built on both sides of the station.
However, terminal stations become very inefficient once traffic increases, because all trains have to pass the junction in front of the station twice, when entering the station and again when leaving. It is therefore necessary to make this junction as efficient as possible.
A simple solution is to use pre signals. If both platforms are in use, the entry signal (yellow bar) shows "Stop!". Any approaching trains wait in front of the junction until a platform becomes available.
A much better solution is the usage of path-based signals.
For larger terminals with many platforms, the junction becomes a neuralgic point. Only one train can be in this signal block, all others have to wait. The solution for this problem are PBS signals. Now, several trains can enter the block simultanously, as long as they can find a path through the junction towards their destination. To further improve efficiency, there are additional tracks to give trains more choice concerning their ways through the junction.
The optimal signalling solution for terminal stations in OpenTTD are path signals. There is a signal in front of every platform, through which a train reserves its path. When leaving, the train waits at the signal, until it can reserve a path through the junction. The signal on the other side of the junction is one-way so trains cannot leave on the wrong track.
With programmable signals, one can force trains to prefer certain tracks by setting signal states appropriately. In this example (originally posted by MB on the German forums), trains should always use the platform to the left if possible, in order to allow a quicker journey out of the station. To do so, the signal in front of the platform to the right needs to be programmed so that it shows red whenever the signal in front of the other platform shows green.
This results in the programming as seen here: "Signal status NO is green (coordinates of the signal in front of the left platform)". This allows access to the right-hand platform only if the platform is free and the platform to the left is occupied at the same time.
