Metro Rings and Loops

Metros and subways are abundant throughout the big cities of the world. 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.

33 circular lines or pan-shaped loops are currently operational in 26 cities. London, Sydney and Tokyo have two rings each, Madrid and Singapore even three. The average circumference of existing rings is about 20 kilometres, ranging from as little as 3 kilometres (Miami, Chicago) to some 49 kilometres (Seoul). 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.

Contents:
Terminology and Types of Rings
Usability
Facts and oddities
Complete list of ring lines
Reference

Terminology and Types of Rings

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. Example: Paris.
	Inclinational spiral An oddity that has the only purpose for a metro line 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.

Usability

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 [5]. However, the thing which comes to mind first, one big transfer station in the centre of the circle with all radial lines crossing, is inefficient. The radial lines should instead meet in many transfer stations within the circle. Moscow serves as an excellent example to this idea, see map below.

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 network consisting of a circle line crossed by several radial lines [5].

Moreover, the relatively large number of metro rings currently proposed or under construction seems to prove that circle lines are still a good option to enhance the usability of an existing metro network. Only two cities, Hamburg and Vienna, have given up circular operation by splitting former circle lines into parts currently served by separate lines (Hamburg has corrected its mistake 42 years later). The disjunction of the ring line in Berlin had its cause in the division of the city during the Cold War.

When a ring is shared among different metro lines, this can be unsatisfactory in terms of operation and traffic, as delays can be introduced from the branches and accumulate along the endless route [3]. 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 [7].

Moscow's metro system is an excellent example for the circle-radial network type (click here to see more maps).

Facts and Oddities

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 [1].

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, Stalin 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 left. The planners took this as a sign of Stalin's genius and started building a metro circle line (Kol'tsevaya Line) [8]. Whether this is true or not, Moscow's circle line is colour-coded in 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 that can be used 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 way to prevent confusion is to give clockwise and counter-clockwise trains different line 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'. For example, Line 5 could stand for the circle line as a whole, whereas Line 5a would refer to the counter-clockwise trains and Line 5b to the clockwise ones. 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 in one direction) makes them feel uneasy. This has at least been reported from Madrid [7].

Complete List

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.

Isolated circles

Shared circles

Pan-shaped loops

Abandoned or operationally split circles

Rings under construction

Proposed rings

Terminal loops

Inclinational spirals

Reference

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