Passing loop
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A passing loop (or "crossing loop") is a place on a single line railway/tramway where trains/trams in opposing directions can pass each other. Trains/trams in the same direction can also overtake, providing that the signalling arrangement allows it. A passing loop is usually double ended and connected to the main track at both ends of the station, though a dead end siding, which is much less convenient, can be used. A similar arrangement is used on the gantlet track of cable railways and funiculars.
Ideally, the loop should be longer than all trains needing to cross at that station. If one train is too long for the loop it must wait for the opposing train to enter the loop proceeding, taking a few minutes. Ideally, the shorter train should arrive first and leave second. If both trains are too long for the loop, time-consuming "see-sawing" (or "double saw-by") operations are required for the trains to cross.
On railway systems that use platforms for passengers to board and disembark trains, especially high level platforms, platforms may be provided on both main and loop tracks or just the main track.
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[edit] System of working
[edit] Main and loop
The main line has straight track, while the loop line has low speed turnouts at either end. If the station has only one platform, then it is usually located on the main line.
If passenger trains are relatively few in number, and the likelihood of two passenger trains crossing each other low, the platform on the loop line may be omitted.
[edit] Platform road and through road
The through road has straight track, while the platform road has low speed turnouts at either end.
A possible advantage of this layout is that trains which wish to pass straight through the station can do so uninterrupted (they do not have to reduce their speed to pass through the curve).
Since there is only one passenger platform, it is not convenient to cross two passenger trains.
[edit] Up and Down working
Trains take the left-hand track (for example) in their direction of running. Low speed turnouts restrict the speed in one direction. Two platforms are needed and they can be island platform or two side platforms. With an Up and Down loop, overtaking is not normally possible as some of the necessary signals are absent.
Crossing loops using Up and Down Working are very common with British practice. For one thing, there are fewer signals if the tracks in the station are signalled for one direction only, and also there is less likelihood of a collision caused by signalling a train onto the track belonging to the opposing train.
The speed restriction in one direction can be eliminated with higher speed turnouts, but this may require power operation as the longer and heavier high speed turnouts may be beyond the capability of manual lever operation.
[edit] Simultaneous crosses and passing lanes
If a crossing loop is several times the length of the trains using it, and is suitably signalled, then trains proceeding in opposite directions can pass (cross) each other without having to stop or even slow down. This greatly reduces the time lost by the first train to arrive at the crossing loop for the opposing train to go by. This system is also referred to as a dynamic loop. For cost reasons, the length of these passing lanes have been reduced from 14km to 7km.
In the AusLink project for the Junee to Melbourne line, roughly every other section of single line will be duplicated to provide so-called passing lanes. About 220 km of the 450 km line will be duplicated.
In Sweden, the passing loops are generally 750 m long, made for cargo trains. Passenger trains are usually much shorter, at least on most single track lines, less than 200 m. The signalling system now allows two passenger trains to cross without stopping, but one has to slow down to 40 km/h, because of the limited length of the loop and the sharp curves in the switch points.
For Norway an investigation has been made about future high speed railways, using 250 km/h as cruise speed. The most promising link would be a new Oslo-Trondheim railway, which is suggested to be single track along a 370 km long route. It is suggested to have about 15 km long passing loops, more like 15 km double track, located about 80 km apart. This would enable passing in 160 km/h, but there could be only one train per hour per direction on the rail line. See also High-speed rail in Norway.
[edit] Automatic operation
Many crossing loops are designed to operate automatically in an unattended mode. Such loops may be track circuited with home signals cleared by the approaching train. Some loops have the points in and out of the loop operated manually, albeit more recent examples have so-called self restoring switches that allow trains to exit a loop without needing the change the points.
Other forms of remote operation included Centralized traffic control, where a train controller changes points and signals from a remote office; and driver operated points, which enable train crews to use a radio system to set the points from a distance.
[edit] Gradients
The design of crossing loops may have to be modified where there are severe gradients that make it difficult for a train to restart from a stationary position, or where the terrain is unsuitable for a normal loop.
One oddity was Dombarton, New South Wales in Australia, where the crossing loop built to divide a long single line section on an extreme 1 in 30 (3.3%) gradient. The "loop" was built as a miniature zig-zag with the lower switchback on one side and the upper switchback on the other side, with a dive tunnel under the through track connecting the two.
[edit] Line capacity
Line capacity is determined by the spacing time-wise of the crossing loops. The longest section between successive crossing loops will, like the weakest link in a chain, determine the overall line capacity.
[edit] Short loops
It is best if all crossing loops are longer than the longest train. Two long trains can cross at a short loop using a slow so-called see-saw process.
[edit] Accidents at crossing loops
The legendary train driver (U.S.: engineer) Casey Jones was killed in an accident in 1900 involving trains too long to cross at the crossing loop at Vaughan, Mississippi. The trains trying to cross were occupying both the main and loop tracks, and in addition, the train doing the see-saw was standing outside station limits. Jones was traveling fast in order to make up lost time, and did not stop in time to avoid a collision.
- Exeter, Australia
Exeter crossing loop collision occurred at Exeter railway station in New South Wales, 1910s, in fog; one train too long for loop.
- Geurie, Australia
Geurie crossing loop collision - train in loop standing foul of main line, causing collision.
- Violet Town, Australia
Violet Town railway disaster - driver dies approaching crossing loop, and goes through loop without stopping, colliding with opposing freight train.
- Hines Hill, Australia
- Zanthus, Australia
Zanthus train collision - co-driver operated loop points prematurely.
[edit] Other accidents
- Dugald, Manitoba
- April 12, 1909 – Gary, Indiana, United States: A westbound Chicago South Shore and South Bend Railroad train runs past a meet point and causes a head-on collision with the eastbound train.
- June 19, 1909 – Shadyside, Indiana, United States: An eastbound Chicago South Shore & South Bend Railroad train runs past a meet point and causes a head-on collision with the westbound train.
[edit] Other names
- crossing loop or passing siding - latter used in America
- refuge loop or siding - used on double track lines
[edit] See also
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