An improved signalling technique for very efficient networks and stations
Configuration file: presignals
Command line: -w
Configuration file: extpresignals
Command line: -Xw
These switches introduce a new type of signal to TTD, the "pre-signal." This signal is in some sense similar to an advance signal, but it's not quite the same. For a detailed description of how pre-signals work and in what cases they are useful, please see Pre-signals (detailed).
Note that you need to load signals.grf to be able to distinguish pre-signals from regular signals. See New Graphics for more details.
The idea of pre-signals is quite simple: a pre-signal shows a green light if, and only if, one of the exit signals from the block behind it shows a green light. This means that trains will only enter the pre-signal block if there is a green exit. As a result, the block will only be entered for short times and trains will leave it immediately, allowing for a very efficient layout. This is particularly useful for station entrances, but can also be applied in other cases.
The presignals switch activates automatic pre-signal configurations. Using this options, your signals will be converted to pre-signals and pre-signal exits whenever a train enters a block that qualifies as an automatic pre-signal block. For a definition of what this needs to look like, please see Pre-signals (detailed). Whenever you modify the block (i.e. add rails, remove them, etc.), the automatic signals will revert to regular signals. Note that in no case will your manual settings (see below) be modified.
With extpresignals, you will be able to manually modify each signal, and force it to be either a pre-signal or a plain signal, no matter whether it is in a valid automatic pre-signal block. The way to change them is the same as switching between one-way and two-way signals, but you hold down the Ctrl key while doing so. That is, you select the "buy signal" option from the railway menu, press the Ctrl key, and click on the signal you want to change. Its appearance should change as a result, and you can verify the setting by using the query tool.
You can also remove your manual selection by choosing "buy signal" and the bulldozer, and then clicking on the signal while holding Ctrl. In other words, you do as if you would remove a signal from the track, but with the Ctrl key pressed.
You can also specify both options, in which case you will get automatic signals but can override them using the manual selection.
Please read Pre-signals (detailed) for a more detailed description of how pre-signals work and what types of setups are useful.