Double junction

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A double junction is a railway junction where a double-track railway splits into two double-track lines. Usually, one line is the main line and carries traffic through the junction at normal speed, while the other track is a branch line that carries traffic through the junction at reduced speed.

A number of configurations are possible.

1 Diamond
- 1.1 SPAD protection
2 Switched Diamond
3 Ladder
- 3.1 SPAD protection
4 Single Lead
5 Diamond and Wide Centres
- 5.1 SPAD protection
6 Flyovers
- 6.1 SPAD protection
7 References

[edit] Diamond

The simplest and oldest arrangement consists of two turnouts and a diamond crossing. Because the diamond needs to be relatively coarse, say 1 in 8, the curve radius is necessarily small, leading to a speed of perhaps 25 km/h. This type of junction is common on street-running tramways, where speeds are quite low and junction must fit in to the available road space.

[edit] SPAD protection

A train from R to P with 12 points reverse is protected from a train from P doing a SPAD by 11 points also lying reverse.

A train from P to Q is not protected from a train from R doing a SPAD.

[edit] Switched Diamond

The fixed diamond can be replaced with a switched diamond, which eliminates the gap in rails at the K-crossing. However, switched diamonds are not a perfect solution to the K-crossing probem, as the switches are very coarse compared to the finer switches of a turnout, and require high maintenance.

[edit] Ladder

An improved junction replaces the diamond with turnouts, which can be of as fine an angle as possible, so that this junction can carry branch traffic at high speed. This configuration assumes power operation of the points, as high speed turnout are generally not suitable for mechanical operation. The high speed turnouts may require more than one point machine each. The turnouts can have no superelevation while the curve in the branch can; therefore the radius in the turnouts must be greater than the radius of the curve in the branch.

[edit] SPAD protection

Essentially the same as for a Double junction with Diamond.

[edit] Single Lead

A single lead junction is used where traffic density is lower, and moves one of the turnouts on the main line onto the branch. This may reduce the number of turnouts on the main line that are subject to heavy wear and tear. The diamond crossing is inherently a heavy wearing component due to the gaps in the K-crossings, and this configuration eliminates the diamond. However, unlike in the ladder, branch trains in opposite directions can collide head-on at 32 if either one passes a signal at danger (SPAD). This has contributed to fatal accidents, e.g. in the UK at: Glasgow Bellgrove on 6 March 1989 and Newton on 21 July 1991 ^[1].

[edit] Diamond and Wide Centres

A double junction with a diamond can have its speed limit raised if the track centres are widened to allow for a coarse-angled diamond crossing with fine-angled turnouts. Examples are at Cabramatta, Wootton Bassett.

[edit] SPAD protection

Essentially the same as for a double junction with diamond.

[edit] Flyovers

A double junction can be grade separated so that there is no flat crossing, reducing conflicts and reducing congestion. Flyovers require a lot of space both lengthwise and crosswise, and cannot always be built. Flying junction example at Aynho Junction. Diving junctions such as at Chatswood are a variant.

[edit] SPAD protection

Because the diamond crossing or equivalent is eliminated, one of the potential SPAD hazards is also eliminated, leaving just the merging junction hazard.