Pre-Signaled Roundabout

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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
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

The Pre-Signaled Roundabout is a combination of ideas. The primary idea is the Right-of-way depots which uses signals to give priority to trains travelling on the main lines. The second idea is your basic Roundabout. One of the flaws of the Roundabout is that it can only handle 2 trains at a time. This slightly larger version can handle 4 trains. Additionally, trains turning right do not need to enter the Roundabout, instead passing by on the outer paths. So even if 4 trains are in the inner loop, other trains can still make right turns. The second flaw in a Basic Roundabout is that there is no preference given to exiting trains or entering trains. Using the Presignal Hack First described in Right-of-way depots, we are able to give preference to trains exiting the loop. This greatly reduces the chance of a traffic jam. (I'm pretty sure the only condition that will cause a jam is four left turning trains entering the loop at exactly the same time.)

Pre-Signaled Roundabout


Junction Criteria
CostSteped
Construction DifficultyModerate
TunnelsNo
BridgesNo
Breakdowns
Broken down trains affect whole junction.
Sharpest Turn1
HillsNo
Split Before MergeNo
Direct PathsNo
Natural TurnsNo
DeadlockYes
LoopsYes
Signal type Pre-Signal
  • Traffic jams unlikely but possible (large trains).
  • Simple to start, complexity added later.
  • Complex signals may be hard to get right.
  • Area: 15 x 15


Variations

Starting loop for early game

An additional benefit is that this is a junction that can be built very simple to start with, and then built up. For example, you can start with a square version of the basic Roundabout, and later on convert it into the Pre-Signaled version. This makes it very cheap to start with, and later when money is available the job can be completed.

Pre-Signaled Roundabout (advanced)

This is a little harder to build, but seems to be easier to find room for.

Pre-Signaled Roundabout (proved)

This is easy to build. A train can enter if and only if there is no trains on the two following steps. It has been proven that no deadlocks (or traffic-jams) can occur. This verification has been done using Petri Nets. [1]

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