One of the most interesting features of TTDX is the signalling. In many other railroad games, signals are simply lacking, instead trains randomly choose where to go and if two trains get on the same track they can just go through each other without being damaged. In TTDX, placing signals correctly is almost as important as building the track in the first place. Without signals, one cannot build complex railway networks, which are essential for high profit.
Building signals
Signals are built using the appropriate tool from the menu. At the first click, the signal will be built, clicking this signal again while using the signal building tool changes the direction. After placing the actual signal, one can change the signal type using the graphical interface.
When holding Ctrl while clicking a signal as when to change the signal type, the GUI window opens. One can select the signal type by simply clicking the appropriate icon, with the greyed icon marking the current type.
In order to fully use the signal menu, one needs to download and add an additional graphics set: Download from tt-forums.
Terms and descriptions
To understand the theories in the following paragraphs, it is necessary to know some terms:
- signal block
- A signal block consists of all pieces of track between entry signals and exit signals. There can be up to 255 signals for a signal block, which should be more than enough. There can also be blocks without entry/exit signals, so that trains cannot enter/leave the block, but these setups do not any have practical relevance.
- entry/exit signal
- Entry signals are all signals that lead into a signal block, exit signals lead out of the block (this is not about the type of signal, just about the fact whether the signal is at the beginning or the end of the block).
Exit from one block means entry into the next block. That means a signal is an entry and an exit signal at the same type, depending on the block in question.
In case this was too complicated and theoretical, an example from road traffic may help: A signal block can be compared to a busy intersection. On each side there are traffic lights that manage who may enter the intersection, so that crashes are avoided. One may only enter the intersection by passing a green traffic light (so the traffic light can be seen as the entry signal for a signal block). Now one may continue down the road up to the next traffic light. There one leaves the block, the second traffic light acts as exit signal for that section.
Signals divide the rail network into disjoint sections (signal blocks), which ensures that all trains have sections reserved exclusively for themselves in order to avoid crashes. These signal blocks may be several miles long in the real world. This length is linked to the braking distances of the trains, which can easily reach a mile or more, depending on the weight and speed of the trains.
Examples
This is a very simple signal block. The signal to the left leads into it, the other one leads out. The tracks to the left and right of that block form additional signal blocks.
Here, one track branches into three. This is the common case at junctions, where signalling becomes important.
The inverse is also possible, four tracks merge into one here. In such a case signals are vital for directing traffic.
A signal block does not have to consist of just railway tracks. Here the station forms one signal block, guarded by a two-way signal to allow entry and exit.
Depots are a special case, as they have a built-in invisible signal. Therefore the rails in front of the depot are a different signal block than the depot itself.
Behavior
Signal aspects
In TTDX, a signal can either show "Stop!" or "Go!". This is true for all signal types, also for presignals and path-based signalling! Both enhancements only change the conditions that determine whether a signal should switch between the two states. A light signal shows its state by a green or red light. Semaphores raise or lower their arms.
Example for semaphores and light signals, each showing "Go!" (left) and "Stop!" (right).
Passing a signal
A train will never, under no circumstances cross a red signal (the player may enforce it manually, though). It will either try to find another way, or it will stop at the signal, waiting for it to turn green. Whether a signal can be passed from the wrong side depends on the signal type (this is described in detail on the corresponding section describing the signal type). If a signal has not turned green after a certain amount of time, a train waiting there will turn around and try to find another way. The waiting time can be set (which should not be necessary in general, the presets should be fine).
Pathfinding
Before discussing the influence of signals on the behavior of trains it is important to take a short look at their pathfinding. Think of a moving train as the result of two processes: pathfinding tells the train where to go, and the signals tell the train when to go. Usually, both processes are connected in various ways, i.e. as soon as you build junctions and signals a train will not move without the results of both processes.
Pathfinding in TTDX and TTDPatch roughly works as follows: The train itself knows its own position and the position of its destination. Every time a train arrives at a switch, it will look ahead to choose where to go. This look-ahead is limited, it will stop after seven junction tiles or 64 squares, whatever happens first. If pathfinding finds a route to the destination within that look-ahead, the train will take the route leading there. If the destination is farther away, the train will take the way that will take it closest to the destination. However, this may result in trains going the wrong way because the path takes them nearer, but not to their destination, and they may get stuck somewhere. In this case, use waypoints to improve your network.
One important aspect about the influence of the signals on pathfinding is the subtle, yet important difference between one-way signals and two-way signals (no matter whether they are standard, presignal or PBS ones). The following examples will show this. The driving direction is always from the left to the right, so the signal to the left is the entry signal, the signal to the right is the exit.
In this picture two junctions are shown, the only difference being the exit signals. Trains will therefore behave differently. Two-way signals tell a train that there are several equal possibilities to choose from. This means if one exit shows "Stop!", the train will take the other one. This enables trains to choose any available platform on a station, but do not use this on switches where tracks will go into different directions!
One-way signals do not allow the train to choose. If the exit that a train has chosen shows "Stop!", it will not go anywhere else, but stop in front of the signal, waiting for it to switch to "Go!".
This picture shows the behavior. The left train waits at the one-way signal showing "Stop!", ignoring the other one to the left that shows "Go!". The other train chooses the free platform, since it is allowed to choose from all possible two-way signals.
If the shortest way to the destination is marked by a one-way signal, trains will ignore any two-way signals which lead to the destination as well. In the case of the image, the train has chosen the track towards the middle platform and has to wait at the signal, although the other platforms are free.
If the shortest way to the destination is marked by a two-way signal, trains will look for alternative paths to their destination. Trains prefer to take the path to their right if it is free, regardless of the kind of signal placed there.