Why should one bother with building a complicated network with tons of signals and stuff? This question is asked by most newcomers to the game. The answer is rather simple: You will run out of space for building a dedicated track for each train. And with a simple double tracked line it is easily possible to have a dozen trains running on it. When having to change a connection or add access to the next factory, this becomes much easier compared to building a whole new connection. In the long run, you save lots of money that can be invested in vehicles (aka moneymakers).
There is one disadvantage, though. Trains in badly designed networks might get lost or block each other. But these problems can easily be circumvented when keeping some simple basics in mind when building the network. And that is what this page is about.
Single line
Single line setups are the cheapest way to connect two stations. As the name suggests, there is only one track which is used in both directions. This severely limits the number of trains that can be used on such lines, so these setups are useful only in the beginning when you have little money, or for branch lines that are used by a very small number of trains.
This most simplistic form can only handle one train that travels between both stations. Between both ends of the line additional stations can be built, but as long as they do not have more than a single platform and appropriate signalling, the capacity of the line is still limited to a single train.
Adding one platform at one end allows two trains to use the line. At any given time, one train waits at station one, while the other one travels between both stations.
This setup can be used by three trains. Two trains are waiting, one at each station, and the third one travels between both stations. This system only works with PBS, because this does not allow trains to go towards a two-way signal showing "Stop!". Without PBS it could happen that both platforms are used and a third train approaches the station, blocking the line because it cannot enter the station.
This setup can be expanded, with each new platform allowing one more train. However, as the connection between the stations remains a single line, this is not very useful as trains would wait for a green signal most of the time. It is much more useful to add passing places.
Shared tracks
Usually, lines will meet at stations, or there are parts which will be used by several trains together. It is necessary to place the signals correctly for this to work, as shown in the next images.
Both lines to the left and right share the central track. The two-way signals make sure that only one train can use this central section, separating it from the rest of the line. However, this only works for two trains, one per line!
Instead of sharing pieces of track, trains can share a station just as well. Signalling works in the same way. Of course, there is still the limit of only having one train per line, so two trains in total.
One-track lines can also share larger stations. Still, only one train per line is allowed, so that the tracks leading away from the station are not blocked. In this setup, three trains can be used. This would also work without path based signals, but it would be less efficient.
This layout can be extended easily, either by adding further one-tracked lines, by adding tracks to a line, or by enlarging the station.
Passing places
Bigger stations at the end of the line did not help much to raise the capacity of the line, as long as trains could not pass each other outside the station. To make this possible, passing places (sidings) have to be built.
To make it work right you have to make sure that the passing area is long enough for your trains, so that they do not stick out at the end. You also have to be very careful about signalling here: If trains can block each other at a passing place, they eventually will do just that, forcing the player to manually solve the problem. Still, this can be prevented by adhering to a simple rule:
The following example shows how not to do it and what will eventually happen. Please do not built it this way, the correct solutions are shown further down.
The classic idea with standard signals looks like this (it would be possible with two-way signals, but that is not the point). Trains have to wait at the passing place until the single-tracked line becomes available. Without further passing places to the left or right, three trains can be used safely, one waiting at the passing place, the others using the single-tracked line to the left and right.
As long as there are only three trains, nothing can go wrong. However, one might get an unpleasant surprise when extending the setup. The system can run into deadlocks, as shown in the picture. All trains are blocked, the user has to intervene. Such problems can arise if there are further passing places behind the single-tracked block. So, just one train too many at the wrong place will lead to utter chaos.
Here we see the reason for the blocking. Behind the single-tracked block there is another passing place. Now this is what happened: Passing place 2 was already fully occupied, the single tracked connection was available, and in passing place 1 a train was waiting to go to passing place 2. Since the track is clear, it can go. However, it cannot enter passing place 2, because it is already occupied. Thus, the train blocks the track. The same thing might happen on the other side, and thus we have a fine traffic jam.
So how can the rule stated above be fulfilled? One could use pre signals. In TTDPatch one could possibly program the signals fittingly (which mimicks the pre signals). But this could still go wrong when having larger passing places or combining lines. The only solution that is guaranteed to work is the usage of path based signals.
The two path signals regulate traffic at the passing place. A train reserves a path to the signal at the end of the passing place. As soon as it enters the passing place, the track behind it becomes available, a train waiting to go in the other direction can now reserve the track and start its journey. A train following the first one will have to wait until the first train has left the passing place, as this makes the path available for reservation. This guarantees that train cannot block each other.
Instead of having tracks in a passing place, one could also build a station there. One can also increase the size of the passing place / station by adding further tracks to the left and right. However, increasing the size is not that useful, one should rather add a second track to the line.
Shared tracks revisited
As already briefly discussed in the part about one-tracked lines, adding passing places into the setups might require some changes to signal placing. The examples from above are repeated here with explanations what has to be changed.
The path signals at the passing places replace the signals that had been placed directly at the beginning of the shared line. This does not change behaviour, trains reserve their way before leaving the passing place towards the shared track.
If there is a station at the end of a single-tracked line and there are passing places before that part, one needs to place a two-way signal in front of the station. Without this signal the path reservation runs into problems, since trains can only correctly reserve a path from the passing place into the station. The way back is only reserved after the train has reached the station. However, it might happen that the passing place is already in use, so that the train cannot reserve a path from the station to the passing place. So in order to prevent crashes, the game will stop the train there, forcing the player to solve the problem. The two-way signal at the station solves the problem, enabling the train to reserve the paths properly, and when no path out of the station is available, it will wait at the signal.
Sharing stations between several single-tracked lines with each being used by more than one train also requires slight changes in signalling. The signals at the station stay the same, but the ones at the single-tracked line are removed, their task is taken over by the signals at the passing places.
Double line
For large-scale tasks (meaning many trains), single tracks are not enough, because of the increased interference of trains with each other. The capacity of a track that is used in both directions is just too small. The solution is simple: One dedicated track per direction. Unless your trains are all going around a town in a circle, this will result in two tracks and huge capacities or just about any task.
This is the standard double-tracked line. Noteworthy are the signals placed in regular distance . These signals act as automatic signal block (that's the term in the real railways) and increase the line capacity. The closer these signals are placed, the sooner a train can follow, and the more trains can run on the line in the same time. In reality, signals are placed several kilometers apart, in the game a distance of about three tiles is usable very well.
Double-tracked lines have enough capacity for most tasks, even with many trains running on them. In order to create a well-working network, well-working junctions will have to be used. This topic is rather complex, therefore it is discussed on an extra page about junctions and crossings.
In real networks, trains can often switch onto the "wrong" lane (the one for the opposite direction) in order to overtake slower trains. This can also be done in the game, although one should only try it on lines with not too much traffic, because that would only lead to train blocking. This system is especially useful if the trains have a huge difference in speed, i.e. for express trains overtaking slow freight trains.
To allow switching the track one has to use signals which can be crossed in both directions and which allow reserving paths in such a way that a returning onto the correct track is simply possible. This is only possible using through signals. The layout shown here is just one possible solution, it has the advantage that trains cannot turn around, preventing them from getting lost.
