The Advtrains mod for Luanti

3 Interlocking and line automation

Interlocking is a set of equipments employed to prevent train collisions while allowing trains to go to their destinations. This chapter will explain some basic concepts and includes a task at the end that you can try to do yourself.

Please note that this chapter is also intended to be used as a reference, so some things may be explained in a way that is not easy to understand at first. If that is the case, the following links may be helpful, albeit possibly outdated:

• • The online interlocking guide: https://advtrains.de/interlocking

• • Blockhead’s Youtube video showing a simple three-station setup (see exercise below): https://www.youtube.com/watch?v=ylG1vHj4zjg

• • Videos from the Advtrains website: https://advtrains.de/videos/

It is also recommended for new players to read through the entire section to understand various concepts of the interlocking system before following the instructions in the subsections. Section B includes certain examples that may be helpful.

Exercise 2

After you finish reading this section, set up interlocking for the three-station setup shown below.

3.1 Basic concepts of interlocking

Unlike road traffic, trains are not always able to brake to a complete stop within a distance that the driver can see. It is therefore required that the track ahead of a train is secured using technical means¹ in such a way that the train can run on the tracks safely even when the driver has limited sight. Among other things, it is necessary that

• • The train does not run into another train unless this is intended for the purpose of shunting.²

• • Track components (in particular, turnouts) are not modified while the train is running on the tracks.

A path along such tracks, along with the states that various components should be put into for safe operation, is called a route . A route can be set only if the above conditions can be met. The system used for setting routes is called routesetting . After setting a route, track components can no longer be adjusted, and conflicting routes for other trains are not allowed to be set.

Note that setting conflicting routes is not allowed in the sense that the interlocking system would not allow such paths to be set.³ However, this alone does not directly prevent a train from proceeding along a potentially dangerous path. Therefore, the signaling system is needed to make sure that trains only follow routes that are set for the train to travel along. Signals are described in detail in section 4.

Note that there is a subtle but important difference between rail and road traffic. A rail signal showing that the route ahead is clear indicates that the train may safely proceed to the next signal and that there is no obstacle along the route. A green light in road traffic does not guarantee this - it can be green even in a traffic jam, in which case blindly driving ahead likely has undesirable results.

To guarantee that the route ahead is clear, Advtrains uses a system known as fixed block signaling. Tracks are divided into sections , and each section should not be occupied by more than one train at a time. Routes are then set by reserving the track sections along the route until the train passes through the section. A route can therefore only be set if none of the sections along the route is occupied by or reserved for a different train.

3.2 TCBs

Track circuit breaks (TCBs) are nodes that can be assigned to two-way tracks and indicate the limits of track sections. TCBs have two sides - each corresponding to a direction of the track that the TCB is assigned to - that can be assigned to two track sections.

To assign a TCB to a track:

• • Place a TCB, ideally adjacent to the track it will be assigned to.

• • Right-click the TCB.

• • You will be told in the chat to punch a track to assign the TCB to. Punch the track to assign the TCB to - this track will, in later subsections, be the boundary of track sections.

• • A TCB marker showing the two sides (A and B) will appear on the track you have assigned the TCB to. This will be explained in further detail below.

After assigning the TCB, you can right click it to open the TCB formspec. This will be used in the following sections. Please note that the TCB formspec has a section for side A and one for side B - when following the instructions in the following subsections, make sure you click the button for the right TCB side. The two sides of the TCB are assigned based on the orientation of the tracks. In particular, straight tracks with different orientations can still appear visually the same. For this reason, most graphs in the rest of this chapter will use cardinal directions instead of actual TCB sides, or omit TCB sides if these are irrelevant.

For new players, it is recommended to set up all TCBs before creating new track sections.

3.3 Track sections

In terms of interlocking, track sections are segments of tracks that can be occupied by a train or reserved for a train to pass. The following graph shows TCBs and sections along a line segment. For simplicity, letters are used instead of actual coordinates and IDs.

Section \(a\) begins at the side A of \(P\) (sometimes written as \(P/\mathrm {A}\)) and extends to the west beyond the graph. Section \(b\) is limited by \(P/\mathrm {B}\) and \(Q/\mathrm {A}\), effectively spanning the line segment \(\overline {PQ}\). \(Q/\mathrm {B}\) is not part of any track section and is therefore known as end of interlocking or EOI, as the track beyond it is not part of the interlocking system.

The graph below shows a track segment where \(P\), \(Q\), and \(R\) form a section around the turnout \(T\). Notice that a train entering from \(Q\) cannot directly reach \(R\) without reversing (or vice versa), while a train entering from \(P\) can reach both \(Q\) and \(R\).

To create a track section, punch a track along the section while holding down Aux1. Alternatively, you can also click on “Create new track section” in the TCB formspec. This should create a new track section and add the TCB side to the track section. Other track sides that face the track side you created the TCB with will also be added to the track section. In the graph above, creating a track section at the west side of \(Q\) will automatically add the south-west side of \(R\) and the east side of \(P\) to the track section.

After the track section is created, you can open the track section formspec by right-clicking a track with the trackworker or clicking on “Show track section” in the TCB formspec. The track section formspec allows, among other things, setting route locks, which are passive components that are automatically set when a route including the track section is set.

Note that placing TCBs inside a track section causes the section to be split. Conversely, removing a TCB merges the sections on both sides of the TCB.

3.4 Signals

Each TCB side can have a signal assigned to the side. The signal will then indicate whether the train is allowed to enter the section to which the same TCB side is assigned to. Signals need to be set up in order to be able to set up routes. In the previous graph, in order to let trains move from \(P\) to \(Q\) or \(R\), a signal needs to be set up on the side of \(P\) that faces the track section (i.e. the east side of \(P\)). For an overview of the signals available in Advtrains, refer to section 4. Signals should conventionally face the opposite direction of the side of the TCB so that the driver can see the signals. Signals can be assigned automatically by setting the influence point (see below) for the signal or manually with the “Assign a signal” button in the TCB formspec.

Every signal can optionally have an influence point. This is the point where the aspect of the signal becomes effective, and should be located before the train passes the TCB side that the signal is assigned to. The influence point for signals that are independent of track sections is irrelevant, but should conventionally be close to the signal. The influence point can be set and changed in the signal formspec. This can be opened by right-clicking (and, if the signal is already assigned to a TCB, additionally holding down Aux1) the signal.

3.5 Routes

Routes contain information on where a train goes to, which section should be reserved for the train to pass, and how certain components, such as turnouts, should be set up. Routes are bound to TCB sides, but they usually need to start at ones with a signal assigned. In most cases, routes should also end in such a way that a train leaving the route can immediately enter the next route, with the most common exception being situations where dead ends are involved, such as train yards and termini where the line physically ends (e.g. Stuttgart Hauptbahnhof).

Routes can be created automatically with the Smart Route tool:

• 1. Click on “Smart Route”.

• 2. Click on “Search further” until all desired routes appear in the form.

• 3. Click on “Apply”.

Routes can also be created manually:

• 1. Click on “New (Manual)”.

• 2. Set up passive components (e.g. turnouts) in the section appropriately and lock them by punching, if necessary.

• 3. Punch the next TCB that the train should pass. The track that the TCB is assigned to should have a marker showing “END ROUTE”.

• 4. If the following track section is the last track section in the route and does not require any setup require in step 2 or have more than two TCB sides, click “Finish route at the end of NEXT section”.

• 5. If the route leads to the end of the physical line, click “Finish route at the end of NEXT section”.

• 6. If the route ends at the TCB, click “Finish route HERE”.

• 7. If the route continues beyond the TCB, click “Advance to next route section” and repeat the above steps, starting from step 2.

You can also click “Finish route at the end of NEXT section” at the starting TCB. This can be useful when the last part of a route does not involve setting up further passive components or when the route continues to a dead end.

If the TCB is not considered suitable for route continuation, please check that the side of the TCB from which the train passes the TCB is part of a track section and that the train can reach there without passing a point assigned to another TCB. If you see “Advancing over next section is impossible at this place. End of interlocking.”, please check that the side of the TCB corresponding to the driving direction is part of a track section.

After finishing the route, you are prompted for the name of the route. It is recommended to use a sensible name. You should then see the route formspec. If you don’t, click on the name of the route you just created and click “Edit route”, and proceed to the next section regarding ARS.

3.6 Automatic routesetting

Automatic routesetting (ARS) is a method of choosing a route based on a set of matching patterns. In the previous section, you learned to set up a route, and there is a text input box where you can enter the ARS rules. This area is empty by default, which means that the route is not selected in any case.

ARS rules are delimited by a newline. Each line can be one of the following:

• # <comment> (pattern) Comment

• LN <line> (pattern) Matches trains with the exact line name <line>.

• RC <routing code> (pattern) Matches trains that contain the routing code <routing code>.

• !LN <line> (pattern) Matches trains with a different line name than <line>.

• !RC <routing code> (pattern) Matches trains that do not contain the routing code <routing code>.

• * (pattern) Matches all trains

A whitespace is required before the argument.

ARS rules are designed to short-circuit. The route is selected if a train matches any of the rules.

In some cases, you may want to disable ARS. To do so, click on “Disable ARS” in the signal formspec. Clicking on the same button will enable ARS.

3.7 Automatic working

Automatic working is a system that sets the route after the train passes the signal. To enable automatic working, click on “Enable Automatic Working” in the signal formspec after the route is set. Clicking on the same button again will disable automatic working.

3.8 Station/stop tracks

Station/stop tracks (sometimes simply called station tracks) are special tracks that register a station where a train should stop. The interface of the formspec should be self-explanatory, with a few things to notice:

• • You need to click “Save” after changing the settings.

• • The station code is used to identify the station. The station name is shared among all tracks with the same station code (not vice versa).

• • Station tracks disable ARS for the specific train when the train arrives at the station and enables ARS on the train before departure.

3.9 Considerations for interlocking

The previous sections were mainly theoretical in that the sections mostly introduced new concepts or described how to do things. This section will focus on the practical part of interlocking, in particular certain things to consider when setting up interlocking on a rail line.

3.9.1 Junctions and grade separation

One of the most important considerations when setting up interlocking at a junction is to make sure that multiple trains can go through the junction at the same time when possible.

Consider the junction from exercise 1. Try to interlock the junction based on what you have learned in the last few sections.

The lazy method would be to set up the entire junction as a single track section - you only need 6 TCBs for that. However, a train passing through would occupy the entire junction, even when two trains could use the junction at the same time - for example, a train going from east to west and another one from west to east. This can be solved by a slightly more complicated setup with four sections, each shown below with a different color:

It is also possible to build the junction on multiple levels. This is known as grade separation. Note that grade-separated junctions may appear less realistic because slopes are significantly steeper than their real-life counterparts.

Exercise 3

Build a grade-separated double-track T junction.

3.9.2 Track capacity and deadlock

It is not possible to run an infinite number of trains on a line. Adding too many trains to a line can result in a deadlock, where every train is stuck at a red signal, waiting for the previous train to clear the track section ahead, while the previous train is also stuck at a red signal, waiting for the train ahead of it to leave the section ahead, and so on:

This problem can be trivially solved by removing a train and, if necessary, resetting the track section that the train previously occupied (this can be done in the track section formspec):

However, only the train at the bottom-right can move; other trains have to wait for the trains ahead to leave the section. The ideal situation is that the section length and the number of trains are decided in such a way that trains do not have to stop or slow down between stations:

When actually building a rail line, signals are spaced away much further than in the illustration above. On servers, the distance between signals typically range between about 50 nodes to a few hundred, depending on the speed at which trains run on the line and the number of trains using the line. The distance between signals in station areas usually depend on the size of the station.

Single-track sections can also be a source of deadlocks:

With single-track sections, routes should generally be set up in a way that the train does not stop in single track sections (like train A in the above diagram):

Note that the end of the track (e.g. train B) is an exception, where the train has to stop and reverse. It should be additionally noted that, in this case, preventing deadlocks also reduces the capacity of the line.

3.9.3 Short routes

In some cases, there may be setups with very short routes. In particular, if the length of a route is shorter than the braking distance of the train for which the route is set, the signal at the end of the route will be triggered to set a route.

If short routes are desired, such as at stations, it is generally recommended to disable ARS for the train, such as by using the A0 ATC command, and keep some distance between the influence point of the signal and the point at which the train is expected to stop. Alternatively, you can also limit the speed of the train to trigger the routesetting system later, but this is not the ideal solution in most use cases of short routes.