Priority Merge

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(Cleaned up description and use, removed mention of "main line", moved advanced techniques to end, added high-speed merging segment)
(High-Speed Merging)
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==High-Speed Merging==
 
==High-Speed Merging==
Sometimes you just don't want your trains to stop at a merge at all.  Red two-way signals are the ultimate train pathfinding penalty in OpenTTD; trains will choose any path of track they can to avoid one even if it contradicts their orders and gets them lost.  You can use this to your advantage by disallowing trains from even attempting to access a prioritized merge if there is currently traffic running on it.  By placing a two-way signal at a merge with another track option continuing after it, trains will ignore the merge entirely when the priority signals that it is occupied and continue on their way using the other track option.  Informative picture forthcoming...
+
Sometimes you just don't want your trains to stop at a merge at all.  Red two-way signals are the ultimate train pathfinding penalty in OpenTTD; trains will choose any path of track they can to avoid one even if it contradicts their orders and gets them lost.  You can use this to your advantage by disallowing trains from even attempting to access a prioritized merge (and ending up braking) if there is currently traffic running on it.  By placing a two-way signal at a merge with another track option continuing after it, trains will ignore the merge entirely when the priority signals that it is occupied and continue on their way using the other track option.  Informative picture forthcoming...
  
 
==Right-of-way Merging onto Dual Main Line==
 
==Right-of-way Merging onto Dual Main Line==

Revision as of 04:05, 16 June 2014


Railway Junctions
Junction Criteria

3-Way junctions

Basic 3-Way
Line Merge
Half Cloverleaf
Half Spaghetti
Compact 3-Way
Compact 3-Way B-M Junction
Ultimate 3-way
Half Transmogrified
Braided Junction

4-Way junctions

Basic 4-Way
Roundabout
Cloverleaf
Spaghetti
Star
Complex Star
Branch-Merge
Over-Under
Tetrathorp
High Speed 4-Way Fly-over/under
Pre-Signaled Roundabout
Advanced Roundabout
Right-of-way Roundabout
Improved Roundabout
Transmogrified
Tightlong
Deepblue2k8 4-way Junction

Enormous junctions

Multi-way/multi-track cloverleaves
Dual Tetrathorp
Dual Transmogrified
Dual Branch-Merge
Quad Branch-Merge
Quad Tetrathorp
6-tuple Branch-Merge
8-Way Star
High Speed 4-way

Other junctions

Right-of-way Depot
Dual tunnels
Sideline U Turn
Right-of-way Lane Change
Right-of-way Merge

Track Layouts

4-Way Triple Axial
4-Way Triple Axial 2
Triple Via Mini
High Hill

Unuseful junctions

Crossover
3/4ths junction
Ultimate 3-way junction for 4 tracks

When two lines of track merge into one with high traffic trains will stop and queue up on each line, taking turns entering the merged track. This sounds like a great idea, but because the trains start from stationary each time this significantly slows down overall traffic. Your train network will often be better off giving priority to one line of track (preferably the one with more traffic) and allowing the trains on it to clear the merge at full speed before trains on the other line can access it.

Using specialized signalling you can force trains entering the merge to wait for a gap in order to minimize traffic disruption.

Contents

Basic Priority

The simplest form of priority.

With two-way pre-signals and nothing extra you can force a merging train to stop if there's a train already on the main track directly in front of the combo or exit signal. The disadvantage to this construction is that you can't extend the priority further than the length of two signals.

Priority Merges Using Extra Track

Priority merges using different signal types.

Longer priority can be designated by connecting track to activate the main merging signal upstream. When a train on the prioritized track drives onto a tile connected to the unused track it tells the signal on the merging track to read red. This construction can be made with both path signals and regular block signals on your tracks. The path signal construction needs only a single block signal on the merging track, while the block signal construction requires pre-signalling on the unused track to prevent trains from signalling themselves to stop.


Be careful to construct priority merges to accommodate your shortest train's length. If you have too much space in the middle, a train can briefly occupy a section of track without activating the merge signal, allowing a train to merge and disrupt traffic. This can be prevented by splitting longer priority into segments.

A flawed priority merge with too much un-prioritized space. Notice the merging signal is green when a train on the main track is in the very center.
Longer priority merge fixed by splitting the priority into segments. The merging signal will read red no matter where a train is on the priority

High-Speed Merging

Sometimes you just don't want your trains to stop at a merge at all. Red two-way signals are the ultimate train pathfinding penalty in OpenTTD; trains will choose any path of track they can to avoid one even if it contradicts their orders and gets them lost. You can use this to your advantage by disallowing trains from even attempting to access a prioritized merge (and ending up braking) if there is currently traffic running on it. By placing a two-way signal at a merge with another track option continuing after it, trains will ignore the merge entirely when the priority signals that it is occupied and continue on their way using the other track option. Informative picture forthcoming...

Right-of-way Merging onto Dual Main Line

One method of merging onto dual main line without disrupting traffic.

All signals are combo signals except at points A and B which are entry signals, and the top right, which are exit signals. Adjust the length of the signal chain according to train length and speed.

Trains joining at point A must wait for the presignal chain along the inside lane to be clear. If a train arrives at point B before train A is out of the way then it will switch to the outer lane (at no speed penalty), but only if the presignal chain for the outside lane is clear. If it is not, the presignals at point B ensure that train B will take the line which clears first.

The connection of the presignal chains at point C is optional. It allows A to continue if the presignal chain for the outside lane is clear, even if the inside lane is not, as the train on the inside will probably be able move to the outside to let A out. When the lane-change is north of Signal B, A will always wait for the inside lane to clear, regardless of the state of the outside lane.

Other uses for this include Right-of-way Lane Change and Right-of-way Depot.

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