A double junction is a

railway junction A junction, in the context of rail transport, is a place at which two or more rail routes converge or diverge. This implies a physical connection between the tracks of the two routes (assuming they are of the same gauge), provided by ''points'' ( ...

where a

double-track railway A double-track railway usually involves running one track in each direction, compared to a single-track railway where trains in both directions share the same track. Overview In the earliest days of railways in the United Kingdom, most li ...

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 A branch line is a phrase used in railway terminology to denote a secondary railway line which branches off a more important through route, usually a main line. A very short branch line may be called a spur line. Industrial spur An industr ...

that carries traffic through the junction at reduced speed. A number of configurations are possible.

At grade

Diamond

The simplest and oldest arrangement consists of two turnouts (points) and a fixed 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 . This type of junction is common on street-running

tram A tram (called a streetcar or trolley in North America) is a rail vehicle that travels on tramway tracks on public urban streets; some include segments on segregated right-of-way. The tramlines or networks operated as public transport are ...

ways, 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 single signal box.

Signal passed at danger A signal passed at danger (SPAD), known in the United States as a stop signal overrun and in Canada as passing a stop signal, is an event on a railway where a train passes a stop signal without authority. In the United States and Canada, this ...

(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.

Switched diamond

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 operated

point machine A point machine (also known as a point motor, switch machine or switch motor) is a device for operating railway turnouts especially at a distance. Overview In the earliest times, points were operated manually by levers. Gradually, these were cen ...

s are used.

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 turnouts 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. 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. ;SPAD protection Essentially the same as for a double junction with diamond. Examples include

Harris Park railway station Harris Park railway station is located on the Main Western line, serving the Sydney suburb of Harris Park. It is served by Sydney Trains T1 Western Line T2 Inner West & Leppington Line and T5 Cumberland Line services. History Harris Park ...

, Sydney.

Single lead left hand

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 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.

Risk

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.Hall, Chapter 6: Hidden Dangers - Single Lead Junctions These risks can be reduced by

trap points Catch points and trap points are types of turnout which act as railway safety devices. Both work by guiding railway carriages and trucks from a dangerous route onto a separate, safer track. Catch points are used to derail vehicles which are ou ...

, ATP or TPWS.

Wear and Tear

The diamond crossing is a high wearing and undesirable component: * gaps in the K-crossings, and V-crossing. * vibration at gaps caused by wheels jumping over these gaps. * small parts which allow the joins to bend. * insulated block joints, if needed for

Track circuit A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters. Principles and operation The basic principle behind t ...

s, create more joints to bend. * inconvenience when trying to tamp the track. * inconvenience of speed limits to reduce vibration. * difficulty of obtaining "track possession" to allow maintenance to be carried out. * often the existing tracks do not have space to allow the alternative to be built as an afterthought. * when starting on a "green field" site, the opportunity to install two high-speed turnouts to replace each diamond should not be missed. * the insulated block joint problem can be reduced if

axle counter An axle counter is a system used in railway signalling to detect the clear or occupied status of a section of track between two points. The system generally consists of a wheel sensor (one for each end of the section) and an evaluation unit for c ...

s can replace the track circuits over the crossings. * switched diamonds are only suitable for limited range of crossing angles, and they have their own problems. ** say 1:5 (too coarse) to 1:10 (too coarse). * the tracks through diamonds needed to be co-planar, and if not poor geometry and poor speeds may result. * the legs of the diamond should be straight if possible. *

Trains (magazine) ''Trains'' is a monthly magazine about trains and railroads aimed at railroad enthusiasts and railroad industry employees. The magazine primarily covers railroad happenings in the United States and Canada, but has some articles on railroading els ...

devoted an issue to diamonds, which, in their opinion, are NOT a railroad's best friend ** where traffic over a diamond crossing is heavier in one direction than the other, special measures can be taken to improve performance in the heavier traffic direction. See Trains.

Single lead Right Hand

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.

Diamond and wide centres

A double junction with a diamond can have its speed limit raised if the

track centre The track spacing is the distance between the track centres of double-track railway lines. There are standard distances derived from the standard loading gauge in a country. For high-speed trains and in tighter curves that distance needs to be i ...

s 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). 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. ;SPAD protection Essentially the same as for a double junction with diamond.

Scissors crossovers

Fast Speed

The Channel Tunnel has at its entrances a scissors crossover implemented with two turnouts replacing the usual diamond crossing in the middle. This is only possible because of the wide

s 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 The track spacing is the distance between the track centres of double-track railway lines. There are standard distances derived from the standard loading gauge in a country. For high-speed trains and in tighter curves that distance needs to be i ...

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. Rail Scissors Crossovers on Channel Tunnel SCCT 2

Rail Scissors Crossovers on Channel Tunnel SCCT 2

Medium Speed

Were this junction be made with a

Diamond crossing 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 ...

in the middle, speeds through the turnout would be much reduced, and wear and tear on the diamond would be high. Rail Scissors Crossovers on Channel Tunnel SCCT Slow

Rail Scissors Crossovers on Channel Tunnel SCCT Slow

Very slow Speed

At Hornsby the site is very constrained and speeds through the diamond crossing was only 8km/h, increased to 15km/h after a few changes. The site does not permit any further increase, and the restrictions dating from 1890 remain.

Track centre The track spacing is the distance between the track centres of double-track railway lines. There are standard distances derived from the standard loading gauge in a country. For high-speed trains and in tighter curves that distance needs to be i ...

s are 12' (3.66m) throughout. Colour coding is as follows: * Black - unchanged * Red - scissors crossovers * Blue - roads and road overbridges * Purple - other rail tracks. Speed limits * on the straight through the scissors crossovers are 40km/h. * on the curves through the diamond crossing "X" are now 15km/h. * on the curves from S - P2 and P1 to P are 40km/h. Rail Scissors Crossovers Hornsby NSW

Grade separated

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 Aynho (, formerly spelt ''Aynhoe'') is a village and civil parish in West Northamptonshire, England, on the edge of the Cherwell valley south-east of the north Oxfordshire town of Banbury and southwest of Brackley. Along with its neighbour ...

. Diving junctions such as at Chatswood are a variant. Weaver Junction is the oldest flying junction in the United Kingdom and perhaps the world. See also

. ;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.

References

;Notes ;Sources * {{Rail tracks Rail junction types