Signals

From OpenTTD
(Redirected from Path signal)
Jump to: navigation, search


See the Building signals tutorial for an introduction to signal construction.

Signals are useful devices that allow you to control train movement. They are necessary to keep trains from crashing on railway networks with more than one train.

Contents

Block signals

Block signals, as the name suggests, operate based on blocks of track. If the block of track on the opposite side of a signal is occupied, the signal is red. If not, it is green. A block of track consists of all track tiles reachable from a given piece of track without crossing over signals.

Two-way signals

Two-way signal.png
Two-way signals at a loading station

Two-way signals are used in a railway that has trains moving in both directions on the same track. The most common use is at end-of-line stations, where trains enter and exit the same end of the station. In the example at the right, the signals direct the next train to come into the empty loading bay. They also prevent the train from leaving the loading bay if another train is in the way.

If a train has a choice of two or more directions, each with two-way signals on them, it will choose the direction with a green signal. If all signals are red it will pick the easiest direction and wait for the signal to change.

One-way signals

One-way signal.png
One-way signals in a loop

One-way signals limit train movement to one direction. In the example at the right, the signals force the trains to move in a circle. This has two advantages: trains enter and exit stations efficiently, and the track can have more than 2 trains.

When using one-way signals, be sure that they are all facing the correct direction. It is a good idea to watch the first train you run on the newly signaled line all the way to its destination to ensure you haven't made any mistakes.

Click on an existing two-way signal to toggle it to a one-way signal. Click on it again to change its direction (leaving it one-way); the third time will revert it back to a two-way signal.

If a train has a choice of tracks, each with a one way signal, it will pick the track heading towards its destination (i.e. it will wait until the signal on the track heading towards its destination turns green, as opposed to taking whichever signal is green if one of the signals is red). This is in contrast with two way signals.

If a train arrives at the wrong side of a one-way signal, it will immediately reverse.


Pre-signals

A problem - the left train may leave first.
Solve the problem using presignals

The image on the top right shows a setup where entry to a station is controlled using ordinary signals. When at least one platform is empty, this works well, as an incoming train is always directed to a free platform. However, consider what happens when all platforms are full, as in the screenshot. All branches are showing red, so the incoming train picks the easiest path - straight on. But suppose the train on the platform to the right is going to be in the station for some time. Meanwhile, the train on the left leaves. The incoming train has committed itself to a platform and is stuck waiting for a train to depart even though there is now an empty platform available! We need to stop that incoming train before the track splits so that it picks the empty platform when a train leaves. For this, we need to use pre-signals.

Pre-signals are signals that decide on which colour to show, not only by the status of the track immediately beyond, but also by the status of other pre-signals further down the line. Specifically, an entry pre-signal shows a green light if, and only if, there is a green exit from the block behind it. You, as the planner, need to identify which signals are to be the pre-signals and which are to identify exits.

Entry Pre-signals

Entry presignal.png
An entry pre-signal shows green as long as there is at least one green exit signal on the following section of track. Otherwise it shows red. This prevents trains from entering the signal block until there is an available exit. Note that depots have a built-in two way signal (it works as an entry pre-signal in a pre-signal set).

If there are no signals designated as exits behind the entry pre-signal, it behaves as a normal signal. This is convenient for bi-directional presignals where only one direction needs presignal functionality.


Exit-signals

Exit presignal.png
An exit signal behaves in the same way as a normal block signal but is necessary to trigger the correct colour on entry and combo pre-signals.


Combo-signals

Station with 1 entry pre-signal, 2 combo-signals, and 4 exit-signals
Combo presignal.png
There is a third type of presignal called the combo signal and simply acts as both an entry and exit signal. This allows you to build large "trees" of presignals as shown on the right.

Building Presignals

To build a pre-signal select the appropriate button in signal selection toolbar.

In previous versions, first place an ordinary signal. Then, with Ctrl held down, click the signal to cycle through the different kinds of presignal:

  • Note that you can build one way pre-signals in the same way as you build ordinary one way signals. Remember: holding Ctrl and clicking toggles the type of (pre)signal, clicking without Ctrl changes the direction of signaling - if interface signals not enabled.
  • Remember not to hold down Ctrl when placing the signal initially or you will end up with a semaphore (see below). Also use Ctrl if you have enabled the signal interface.

Limitations

The problem with presignals...

An important point to note with exit signals is that a green exit signal will trigger a green on the entry pre-signal at the beginning of the block even if it is not actually possible for a train to get to that exit signal because of the track layout (as in the image to the right with a train entering on the bottom track). This can ruin more complicated presignaling setups, so care needs to be taken with planning.

One of the bugs that are noted as "will not be solved" is: lost trains ignore (block) exit signals. If trains are lost, a random direction is chosen at each junction, so they ignore block exit signals, and may easily block junctions with presignals.


Path Signals

Two new signal types in the signal GUI.

There are two new signal types. These signals enable trains to reserve a path through a block until the next signal, before entering the block. If another train wants to enter the block, and succeeds in reserving a path through the block, the path signal authorizes the train to enter the block, even if another track of the block is used at the same time.

Y block among three block signals

Here is an example as illustration

We have here a Y block among three signals. The right track is used by a train. A train arrives from the lower track and wants to go to the left track. A standard block signal considers that all the block is occupied (even if the left track is free).

With two additional block signals, theY has been split into three blocks: right arm, left arm and junction

A solution is to split this block into smaller blocks by adding signals after the junction.

Y block between a path signal and two block signals

On the contrary, the path signal authorizes the train coming from the bottom to enter the Y block, as the path it tries to reserve is free (the left track). No need to split the block into smaller blocks (i.e. no need for signals after the junction).

There are two things you should know about this signal:

  • Place it only where trains can stop and wait without blocking junctions ;
  • This signal only works in one direction. In the other direction, it is either ignored or considered a one-way signal, depending on the signal path (see below).
PS
  • Path Signal: trains can pass through this signal from the back side.
1W PS
  • One-way Path Signal: trains can not pass through this signal from the back side.

Most of the time it will be sufficient to use a default Path signal, as passing a signal from the back is penalised by the pathfinder, however, one-way Path signals might be useful in certain specific cases.

The two new signal types behave a bit differently than standard OpenTTD signal types. The Path signals are red by default, and will only show green as soon as a train can reserve a path to the next safe waiting position on its route. Safe waiting positions are - by definition - in front of signals, depots and track ends. The back of a Path signal is not considered a safe waiting position, and therefore paths are reserved through these signals.

Because the front of every signal is defined as a safe waiting position, you would normally not want to place a signal immediately behind a junction, only in front of a junction. This is because it is only safe for a train to wait in front of a junction. It is not safe for a a train to wait at a signal immediately after a junction before the whole train has cleared the junction, as it would be blocking the junction while waiting, as illustrated in the example below. This is a major advantage against standard OpenTTD signals, where you had to place signals before and after junctions, which caused trains to block junctions while waiting.

Bad: The signal indicated by the arrow is a bad waiting location, as the train in the junction now blocks the otherwise free path of the train coming from the top. There's a second bad waiting location in this picture. Can you find it?
Good: Here there is enough space after the juction for a train to wait. As long as this space is unavailable, the next train will wait before the junction rather than blocking it. This allows the train coming from the top to proceed.


This is one other example of what you can do with those path signals. This works far better than using pre-signals because you can have two trains leaving the station at the same time AND share the same depot (there is no way to make it with pre-signals, because of the block share). In this case there are one-way path signals for trains leaving the station, and simple path signal for trains leaving the depot.

Two exit station. Note that the signals in front of the depot are not required, as depots have built-in signals. However, path signals in front of depots like this give trains leaving the depot priority over trains wanting to enter the depot.


Advanced Settings

YAPP Advanced Settings

There are three new Advanced Settings related to path signals.

One option is to highlight reserved tracks. This option is useful to troubleshoot your path-signaled junctions, as you can see what paths trains have reserved through a junction.

The other two options control how the build signal tool should behave. You can set the signal type which should be built by default when building a new signal with the signal tool and change which signal types should be cycled through on ctrl-clicking an already existing signal.

There are a few more path signal related options which are not available through the Advanced Settings window. What these options are, and how to edit them is covered under Advanced path signal options & features.

Basic Examples

Below are some example track layouts which use the path signals. There are also Advanced track layouts that are not recommended for beginners.

Basic junction

This basic junction now has a much higher throughput.

Basic junction for trains with max length of three tiles. Note the use of normal signals on plain track.

Basic two-way station

With this station layout, trains can use both platforms when coming from either direction.

Basic station.

Feature availability

<0.5

☒

0.5

☒

0.6

☒

0.7

☑

1.0

☑

1.1

☑

1.2

☑

1.3

☑

Nightly

☑

Directing trains

Signals are intended to prevent trains from crashing into each other, and to help them choose between several track sections to the same destination. If you want to direct a train to a particular destination via a certain route, you should use Waypoints.

See Also

Retrieved from "http://wiki.openttd.org/?title=Signals&oldid=79130#Path_Signals"
Personal tools