Passing loop
From Wikipedia, the free encyclopedia
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. The passing loop connects to the main track at both ends of the station, though a dead end siding, which is much less convenient, can be used. 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" 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 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 located on the main line. An example is Clarendon railway station, Sydney on the Richmond line.
If passenger trains are relatively few in number, and the likelihood of two passenger trains crossing each other, 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. Riverstone railway station, Sydney had this arrangement, although one end has been slewed to put the platform on the straight road, and a second platform has since been provided.
While the reasons for choosing this configuration seem to have been forgotten, stations using this arrangement in New South Wales at least, appear to be the more important stations, with significant townships attached.
[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. Examples include Quakers Hill.
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.
With and Up and Down loop, overtaking is not normally possible as some of the necessary signals are absent.
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.
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 220km of the 450km line will be duplicated.
[edit] Single ended
Crossing loops are generally double ended, but occasionally a single ended siding is used for crossing purposes. Molong used to have a short crossing loop, but now a much longer length of a closed branch line (2000m) is used to cross trains.
Gerringong had a single ended siding that required fly shunting to shunt.
[edit] Automatic operation
New South Wales has a long history of crossing loops designed to operate automatically in an unattended mode. Such loops are track circuited with home signals cleared by the approaching train. Points in and out of the loop are operated manually, albeit more recent examples have so-called self restoring switches. Examples include Wirrinya.
[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.
Dombarton, New South Wales was a crossing loop built to divide a long single line section and was located 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] Accidents at crossing loops
[edit] Casey Jones
The legendary train driver 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 travelling fast in order to make up lost time, and did not stop in time to avoid a collision.
[edit] Exeter
- Exeter crossing loop collision occurred at Exeter railway station in New South Wales, 1910s, in fog; one train too long for loop.
[edit] Geurie
- Geurie crossing loop collision - train in loop standing foul of main line, causing collision.
[edit] Violet Town
- Violet Town railway disaster - driver dies approaching crossing loop, and goes through loop without stopping, colliding with opposing freight train.
[edit] Hines Hill
[edit] Zanthus
[edit] Other accidents
- Dugald, Manitoba
- April 12, 1909 – Gary, Indiana, United States: A westbound Chicago South Shore & 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] Trivia
The Cheltenham and Gloucester Railway was really a tramway, operated by horse power. Its line was mainly single track, and crossing loops were placed at frequent intervals within sighting distance of each other, so that drivers could see if the way ahead was clear of opposing trams.
