While metro lines are normally planned to be as straight as possible, a special, very urban kind of infrastructure has emerged in metropolitan areas: the Circle Line.
41 circular lines or pan-shaped loops are currently operational in 32 cities. Beijing, London, Moscow and Tokyo have two rings each, Madrid and Singapore three. Singapore's existing rings belong to people movers in the suburbs, but two real metro rings are under construction. The average circumference of existing rings is about 20 kilometres, ranging from as little as 3 kilometres (Miami, Chicago) to 57 kilometres (Beijing). A couple more rings are currently under construction or proposed.
Most circle lines orbit a city centre, some of them since the beginning of the 20th century in order to connect peripheral railway stations within big cities. Circle lines that run around a city centre can increase efficiency of a transport network as they provide valuable connections between sub-centres of the city while releasing burden from lines that pass through the city centre.
Circle, loop or ring? Circle and loop will be referred to here as types of operation of metro lines. The tracks on which a circle or loop line can run (the 'hardware') will be called a ring. The following types can be distinguished:
Isolated circle A single line circles a ring endlessly. Examples: Glasgow, Moscow.
Shared circle Several lines, one of which circles the ring completely while others use only stretches of the ring. Example: London's Circle Line, which is accompanied by Metropolitan, District and Hammersmith&City Lines.
Pan-shaped loop A line which runs from outside into a ring, around it, and out of it on the same route it came in from. Some loops can be technically operated as circles but aren't (like Chicago's Loop) while others cannot (like Tokyo's O-Edo Line).
Operationally split circle Several lines of which none runs around the entire ring (or loop) though they technically could, either in the past or at present. Example: Vienna.
Terminal loop A small unidirectional turnover loop at an end of a line with or without stations. Examples: New York, Paris.
Spiral A loop without a connecting station. Sometimes used for metro lines to follow a steep geographical incline. Examples: Naples, Tokyo.
False circle (not subject of this document) A circular pattern in a metro network which cannot be circled by trains. Occurs in almost every metro system, e.g. lines 2 and 6 in Paris.
A circle line crossed by several straight radial lines is considered to be the best possible type of metro networks, since it provides the best traffic connections within a city . Planners in London have found excellent geological preconditions for metro construction since the beginning of the tube's history. This enabled them to plan for the optimal network type, and they indeed chose a circle line crossed by several radial lines .
However, the thing which comes to mind first, one big transfer hub in the centre of the circle with all radial lines crossing like spokes, has been proven to be very inefficient. Such a central hub would be overcrowded and have long connecting ways. The radial lines should instead meet in multiple small transfer stations within the circle. Moscow serves as an excellent example to this idea, see map below.
The relatively large number of metro rings currently proposed or under construction also proves that circle lines are a good option to enhance the usability of existing metro networks. Only two cities, Hamburg and Vienna, have once given up circular operation by splitting former circle lines into parts later served by separate lines. Hamburg has corrected its mistake 42 years later. The disjunction of the ring line in Berlin was caused by the division of the city during the Cold War.
When a ring is shared among different metro lines, that can be unsatisfactory in terms of operation and traffic, as delays can be introduced from the branches and accumulate along the endless route . Isolated circle lines, however, can be very reliable: Moscow's metro operators use the Circle Line as a test bed for new signalling and trains because it is has the most reliable schedule of all lines .
Train carriages running on a circular line have to be reversed from time to time in order to prevent the wheels from wearing out unevenly .
Glasgow's metro network consists of a single circular line which runs entirely underground. It is publicly nicknamed 'Clockwork Orange', according to the colour of its trains and the colour of the line in the official metro map. Unfortunately, the metro operator is going to change the colour to blue.
Urban legend in Moscow has it that when Stalin was asked by city planners what he thought of their metro network design, he set his coffee cup in the centre of the blueprint and left the room silently. After the cup was removed, there was a brown circular coffee stain. The planners took this as a sign of Stalin's genius and started building the metro circle line (Kol'tsevaya Line) . Whether this story is true or not, Moscow's circle line is still colour-coded brown in most official metro maps.
Train directions of a circular line can be easily confused by passengers, mainly because there are no terminal stops to label the trains. Every station can be reached by going in either direction, but for the occasional user, chances are 50% to take a time consuming detour. A common way to prevent confusion is to give clockwise and counter-clockwise tracks different numbers, like S41 and S42 in Berlin, or names, like Outer Circle and Inner Circle in Glasgow.
Possibly the best way to name a circle line would be to name it with a number and add an 'a' or 'b'. If, for example, Line 5 stands for the circle line as a whole, Line 5a would refer to the counter-clockwise and Line 5b to the clockwise direction. This would make route descriptions very easy to follow, without looking at maps. Letters 'a' and 'b' can work as memory hooks: just remember that a script letter 'a' is written with a counter-clockwise stroke and 'b' with a clockwise stroke.
Some circular lines are likely to be unpopular among train drivers because the endless tunnel trip, and maybe the steady centrifugal force to the same side, makes them feel uneasy. This has at least been reported from Madrid .
Data provided: City name - line name - circumference - number of stations on the ring - time for a circular ride - inauguration of first section - completion of ring - remarks.
Beijing - Line 2 - 28 km - 18 stations - round trip 43 minutes - opened in 1984 - completed in 1987. Beijing - Line 10 - 57.1 km - 45 stations - round trip 104 minutes - opened in 2008 - completed in 2013. Delhi - Ring Railway - 21 stations. Detroit - People mover - 4.8 km - 13 stations - round trip 15 minutes - completed in 1987 - unidirectional. Glasgow - Circle Line - 10.4 km - 15 stations - round trip 24 minutes - completed in 1896. Madrid - Line 12 ('MetroSur') - 40.5 km - 28 stations - round trip 75 minutes - opened in 2000 - completed in 2003 - in southern suburbs. Madrid - Line 6 - 23 km - 28 stations - round trip 67 minutes - opened in 1979 - completed in 1995. Moscow - Kol'tsevaya Line - 19.4 km - 12 stations - round trip 29 minutes - opened in 1950 - completed in 1954. Moscow - Moscow Central Circle, Line 14 - 54 km - 31 stations - round trip 84 minutes - opened in 1908 - completed in 10 Sep 2016 - Initially the freight and passenger Ring Railway, later used for freight only, rebuilt for passenger use again. Singapore - Punggol LRT - 10.3 km - 15 stations - opened in 2000 - completed in 2005 - People mover above ground. Singapore - Sengkang LRT - 10.7 km - 14 stations - opened in 1998 - completed in 2003 - 8-shaped people mover above ground. Tokyo - Yamanote Line - 34.5 km - 29 stations - round trip 64 minutes - opened in 1872 - completed in 1914 - Suburban metro.
Berlin - S41/S42 - 37 km - 27 stations - round trip 62 minutes - opened in 1871 - completed in 1877 - Suburban metro, circular operation suspended 1961-2002. Charleroi - all lines - 4.1 km - 8 stations - round trip 14 minutes - opened in 1983 - completed in 2012 - all lines running around the circle. London - Circle Line - 22.5 km - 27 stations - opened in 1863 - completed in 1884. Miami - People mover - 3 km - 8 stations - completed in 1986 - Elevated, rubber-tyred, automated. Nagoya - Meijo Line - 25 km - 28 stations - opened in 1965 - completed in 2004. Osaka - 21.7 km - 19 stations - opened in 1874 - completed in 1898 - Suburban metro. Oslo - Lines 4, 5, 6 - 13 km - opened in 1898 - completed in 2006. Seoul - Line 2 - 48.8 km - 43 stations - round trip 84 minutes - opened in 1978 - completed in 1984. Seville - Cercanias C4 - 5 stations. Shanghai - Line 4 - 33.7 km - 26 stations - opened in 2000 - completed in 2007 - shared with line 3 .
Brussels - Lines 2 and 6 - 18 stations - opened in 1981 - completed in 2009. Bucharest - Lines M1, M3 - 24 km - 16 stations - opened in 1979 - completed in 1989. Chicago - different lines - 3.2 km - 9 stations - completed in 1897 - The Loop. Delhi - Gurgaon line - 5 stations - opened in 2013 - completed in 2013 - Single-track loop. Hamburg - Line U3 - 17.5 km - 23 stations - round trip 38 minutes - completed in 1912 - circular operation was suspended 1967-2009. Incheon - Wolmido monorail - 5.1 km - 5 stations - opened in 2010 - unidirectional monorail. Kiev - Miska Electrychka - 51 km - 15 stations - round trip 87 minutes - opened in 2009 - completed in 2011 - Urban electric railway, operational during rush hours only. Kolkata - Circular Railway - 20 stations - opened in 1984 - Part of the Kolkata Suburban Railway. Liverpool - Wirral Line - 4 stations - suburban line, single-track loop. London - Central Line - 23 km - 14 stations - completed in 1948 - near Hainault (operated as loop in peak hours only). Madrid - Cercanias Line C7 - 16 stations. Melbourne - several lines - 5 stations - completed in 1984 - Suburban metro . Newcastle - Yellow Line - 26 stations - opened in 1980 - completed in 1982. Paris - Line 7bis - 4 stations - opened in 1911 - completed in 1921 - unidirectional loop. Singapore - Bukit Panjang LRT - 7.8 km - 13 stations - completed in 1999. Sofia - M2 - 16.1 km - 15 stations - opened in 1998 - completed in 2012 - Line 2 runs through Serdika station twice: a ring with two panhandles. Sydney - Suburban metro - 6 km - 6 stations - completed in 2000. Tokyo - O-Edo Line - 28 km - 26 stations - round trip 59 minutes - opened in 1991 - completed in 2000. Vancouver - Millennium Line - 22 stations - opened in 1986 - completed in 2002.
Operationally split circles
Copenhagen - S-Tog - 20 stations - completed in 2005 - operationally split. London - Overground - 33 stations - opened in 2007 - completed in 2012 - split at Clapham Junction and Highbury & Islington. Vienna - Lines U4/U6 - 18 stations - operation as a pan-shaped loop 1925-1978. Vienna - Lines U2/U4 - 10 stations - operation as a pan-shaped loop for only 2 weeks in 1981.
This section may be incomplete.
Beijing - Airport Express - 2 stations - completed in 2008 - unidirectional. London - Piccadilly Line - 2 stations - at Heathrow, unidirectional. Los Angeles - Blue Line - 5 stations - at southern end, unidirectional. New York - Line 6 - 1 station - going through the glamourous abandoned City Hall station without passengers, unidirectional. New York - Lines 1 and 9 - 1 station - at South Ferry station, unidirectional. Paris - Several lines - Paris has the largest number of unidirectional terminal loops worldwide, two of them used with passengers: Nation on line 2, Charles de Gaulle - Etoile on line 6. Seoul - Line 6 - 6 stations - unidirectional.
This rare and unusual type of loop can help rail lines to climb steep geographical inclinations. The one in Naples is completely underground and could go unnoticed by passengers if it weren't plotted in the official metro map. A non-inclining spiral will be the Downtown Line in Singapore, where a spiral with 8 stations is under construction in the city centre.
Naples - Line 1 - Inclinational spiral at Vanvitelli and Cilea/Quattro Giornate stations, underground, climbing up to 250 m above the city centre. Tokyo - Yurikamome Line - Inclinational spiral near Shibaura-Futo station, elevated, climbing Rainbow Bridge.
Rings under construction
Bangkok - Blue Line - 48 km - 37 stations - opened in 2004 - to be completed in 2019 - will eventually become a pan-shaped loop. Chengdu - Line 7 - 38.6 km - 31 stations - to be opened in 2017 - to be completed in 2017 - will be an isolated circle. Chongqing - Line 0 - 50.4 km - 33 stations - to be opened in 2017 - to be completed in 2018 - A true isolated circle, aptly named Line 0. Copenhagen - Line M3 (Cityringen) - 15 km - 17 stations - round trip 23 minutes - to be opened in 2018 - driverless, shared with line M4. Delhi - Line 7 (pink) - 58.6 km - 38 stations - completed in 2016 - Inner Ring Road Line. Will be an isolated circle. Guangzhou - Line 11 - 43.2 km - 32 stations. Kaohsiung - LRT - 22.1 km - 37 stations - opened in 2016 - will finally become an isolated circle. Naples - Line 1 - 26 stations - opened in 1993 - to be completed in 2020. Riyadh - Monorail - 3.6 km - 6 stations - to be opened in 2017 - to be completed in 2017 - Around KAFD area (King Abdullah Financial District). Singapore - Downtown Line - 42 km - 34 stations - opened in 2013 - to be completed in 2017 - The downtown spiral with 8 stations in the middle of a 34-stations line will be unique in layout. Singapore - Circle Line - 37.3 km - 30 stations - opened in 2009 - to be completed in 2025 - driverless. Taipei - Line 6 - 52.4 km - 38 stations - to be opened in 2018 - will finally become a driverless, pan-shaped loop.
Baku - Line 2 - 23 stations - opened in 1985 - Line 2 is planned to become a ring line. Chicago - Circle line - 20 km - 22 stations. Hyderabad - Red Line - 84 km - to be opened in 2017 - The Red Line is planned to become a circle. Moscow - Line 4 - outside line 5 as extension of line 4's Mezhdunarodnaya branch, under discussion. Moscow - Line 11, part of line 1 - Large metro ring, approved in the 1960's. Paris - Arc Express - 50 km - 50 stations - round trip 75 minutes - proposed, driverless. Rio de Janeiro - Line 1 - 23 stations - Line 1 is proposed to become a circle. Seville - Light rail line 4 - 19 stations. Toronto - Yellow/Purple lines - completion unclear.
Sydney - Sydney Monorail - 3.6 km - 8 stations - opened in 1988 - dismantled after 30 June 2013.
 Bayman, Bob: London Underground Official Handbook. 2000.
 Fischler, Stan: Subways of the World. 2000.
 Garbutt, Paul: World Metro Systems. 1997.
 Groneck, Christoph: Metros in France. 2006.
 Schleife, Hans-Werner; et al.: Metros der Welt. 1992.
 Schwandl, Robert: UrbanRail.net  UrbanRail.net newsgroup.
 Wikipedia: Moscow Metro.
Latest update to this page: 6 Dec 2016. Database entries can be more recent.
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