Pocket track

A pocket track, tail track, or reversing siding (UK: centre siding, turnback siding) is a rail track layout which allows trains to park off the main line. This type of track layout differs from a passing loop in that the pocket track is usually located between two main lines, rather than off to the side. Found primarily on metro systems, rapid transit light rail networks, and tramways, a 'reversing headshunt' allows certain trains or trams to change direction, even on lines with high traffic flow, whilst others continue through the station.[1][2][3]

A pocket track on the eBART line
CityLynx Gold Line Birney car laying over at CTC-Arena, viewed from the Lynx light rail station, 2016
The station in November 2021 looking north, with the new platform 5 (which is on a turnback siding) on the far left
Melbourne University tram stop has three reversing headshunts in succession, between the two running lines.

Pocket tracks also allow for the short-turning of trains, truncating services at an intermediate station to control train frequency.[4] They are also used at terminal stations to allow for the construction of future extension of a rail track without disrupting existing service.[5]

Turnback Sidings can have a platform on it to permit turning back of trains and letting passengers change train for example at Stevenage railway station where a new platform 5 was built on a turnback siding.[6]

Typically there will be two running lines, one for each direction of travel, and the headshunt will be positioned between the two running lines, linked to both by points. Although most trains will pass through the station and continue in the same direction, an individual train may be directed into the reversing headshunt, before exiting onto the other running line, in the opposite direction of travel. This procedure allows a greater frequency of trains on a city-centre section of the line, and reduced frequency on the suburban sections, by allowing certain trains to shuttle back and forth only on the city centre part, using the reversing headshunts to change direction within the flow of trains.

References
  1. Railway Technical Web Pages, UK. "Glossary: Centre Siding." Archived 2014-10-07 at the Wayback Machine Updated 2011-08-29.
  2. Railway Technical Web Pages, UK. "US-UK Railway Terminology." Archived 2010-11-28 at the Wayback Machine Updated 2011-08-29.
  3. TriMet MAX (Metropolitan Area Express), Portland, Oregon."Types of track layouts." Max FAQs. 2010-12-17.
  4. "Pocket Tracks" (PDF). Metro 2025: Strategic Plan, 2013-2025 (Report). Washington Metropolitan Area Transit Authority. p. 67. Retrieved August 8, 2016.
  5. "Northgate Station" (PDF). Sound Transit. July 2012. Archived from the original (PDF) on October 2, 2015. Retrieved August 8, 2016.
  6. "Stevenage Turnback Siding, East Coast Mainline". Trackwork. Retrieved 2023-04-26.
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