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Adouble junction is arailway junction where adouble-track railway splits into two double track lines. Usually, one line is themain line and carries traffic through the junction at normal speed, while the other track is abranch line that carries traffic through the junction at reduced speed.
A number of configurations are possible.
The simplest and oldest arrangement consists of twoturnouts (points) and a fixed Diamond crossing. Because the diamond needs to be relatively coarse, say 1 in 8,[original research?] the curve radius is necessarily small, leading to a speed of perhaps 25 km/h (16 mph). This type of junction is common on street-runningtramways, where speeds are quite low and junction must fit into the available road space. Because the points are close together, the entire junction can be controlled by the mechanical point rodding of a singlesignal box.
Signal passed at danger (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.
The fixed diamond can be replaced with a switched diamond, which eliminates the gap in rails at the K-crossing, which allows a higher speed if the geometry is poor. However, switched diamonds are not a perfect solution to the K-crossing problem, as the switches are very coarse compared to the finer switches of a turnout, and require high maintenance. The additional ends are also awkward to control unless power operatedpoint machines are used.
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 turnouts are generally not suitable for mechanical operation. The high speed turnouts may require more than one point machine each. The turnouts can have nosuperelevation 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.
Since the ladder type junction requires much more length, diamond type junctions can only be converted into ladder types if there is room and no bridges, tunnels, or platforms in the way.
Examples includeHarris Park railway station, Sydney.
Asingle lead junction is used where traffic density is lower, and moves one of the turnouts on the main line onto the branch. This reduces the number of turnouts on the main line that are subject to wear.
Space permitting, a single lead junction may be a stage towards construction of a higher speed ladder junction. This is shown as blue dashed line on the diagram.
However, unlike in the ladder, branch trains in opposite directions cancollide 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 andNewton on 21 July 1991.[1] These risks can be reduced bytrap points,ATP orTPWS.
The diamond crossing is a high wearing and undesirable component:
A right hand single lead junction cannot protect against a collision should the Q-P train overrun the signal protecting the junction, and this is not ameliorated by reverting to a double junction.
A double junction with a diamond can have its speed limit raised if thetrack centres are widened from say 4 m to say 12 m to allow for a fixed coarse-angled diamond crossing (say 1 in 8.25) with fine-angled turnouts (1 in 15).[original research?]
If the legs of the coarse crossing X are straight and flat, then this arrangement eliminates the need for switched diamonds and their inconvenient moving parts.
The Channel Tunnel has at its entrances a scissors crossover implemented with two turnouts replacing the usual diamond crossing in the middle.[original research?] This is only possible because of the wide track centres caused by the wide spacing of the entrances to the tunnel. The result is high speeds on the turnout routes through those turnouts and minimum wear and tear.
All the turnouts, P, Q, R, S, X1 and X2 are all the same high speed. Track centres need to be at least twice the normal track centres as if there was a hidden track in between.
All the turnouts are identical except for handedness.
Were this junction be made with aDiamond crossing in the middle, speeds through the turnout would be much reduced, and wear and tear on the diamond would be high.
AtHornsby the site is very constrained and speeds through the diamond crossing was only 8 km/h, increased to 15 km/h after a few changes. The site does not permit any further increase, and the restrictions dating from 1890 remain. Track centres are 12 feet (3.7 m) throughout.
Colour coding is as follows:
Speed limits
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 atAynho Junction. Diving junctions such as atChatswood are a variant.Weaver Junction is the oldest flying junction in the United Kingdom and perhaps the world.
