A Balloon loop is a track arrangement that allows a train to reverse direction, and return to where it came from, without having to shunt or even to stop. Balloon loops can be useful for passenger trains, and unit freight trains, such as coal trains.
Balloon loops do not include track layouts where combinations of junctions allow a train reversal, where this reversal is not regularly used.
Balloon loops were first introduced on metro and tram lines. They did not appear on freight railways in large numbers until the 1960s when the modernising British Rail
introduced so-called merry-go-round
(MGR) coal trains that operated from mines to power stations and back again without shunting.
Balloon loops are essential for operating the single-ended trams found in some cities. Balloon loops were also used by the steam trams found in Sydney
, though the loops were mostly removed when double-ended electric trams came into use.
On a balloon loop:
The station is located on the balloon loop. The platform may be curved or straight.
- City Hall and South Ferry subway stations in New York City:
- City Hall station on the IRT Lexington Avenue Line is now closed, although the loop track continues to be used to turn trains. These trains discharge and take on passengers at Brooklyn Bridge, one station to the north (see next section).
- South Ferry is a two-track loop station, with a sharply curved side platform for each track. While both tracks continue to be used to turn trains, only the outer platform remains in service as a passenger station. Due to problems with train length and platform clearance, this station will soon be replaced by a standard stub terminus with two tracks and an island platform, although the original trackage will remain in use for turning trains when necessary.
- Juniper Street Station in Philadelphia, the eastern terminus for the SEPTA Subway-Surface Lines
- Bowdoin Station on the MBTA Blue Line in Boston has a wedge-shaped island platform inside a balloon loop. The station will be closed when 6-car trains begin operating, as those will be too long for the existing platform and there is no room to extend it.
- World Trade Center station on the PATH subway system linking New York and New Jersey.
- Dungeness (Romney, Hythe and Dymchurch Railway) in Kent, England
- The Eurotunnel Shuttle uses balloon loops. However each of the two tracks crosses over at the French end so it is a Möbius loop. This evens the wear on the wheels of the shuttle engines and vehicles, as each set (left or right) spends only half the time as the outer edge of the line.
- Newcastle upon Tyne has a station that trains from London can arrive at in either direction and return to London using the High Level Bridge and King Edward VII Bridge, but this loop-like feature is seldom used.
- Yuen Long of
- Olympic Park, Sydney, Australia Platforms 1 and 4 are for boarding; platforms 2 and 3 are for alighting.
- In some cases, multiple stations lie on a balloon loop.
- Peasholm, on the North Bay Railway in Scarborough, North Yorkshire has a reverse balloon loop, with the "neck" of the balloon facing the buffer stop. The loop is used to allow the locomotive to run round the train and reverse at the same time.
With balloon loop:
The balloon loop is past the station.
- Bad Herrenalb, Albtalbahn, Germany: Train passes loop before arrival
- Bowling Green on New York City's IRT Lexington Avenue subway line currently serves as the southern terminus for 5 service at all times except rush hours, with the South Ferry inner loop (see previous section) used to physically turn trains.
- Brooklyn Bridge–City Hall, also on New York City's IRT Lexington Avenue Line, currently serves as the southern terminus for Lexington Avenue local service (the 6 train), with the City Hall loop (see previous section) used to physically turn trains.
- Dungeness railway station, Romney, Hythe & Dymchurch Railway, Kent, England. Single track, single platform for both boarding and alighting.
- Howard Station on the CTA Red Line in Chicago.
- 69th Street Terminal, the western terminus of the SEPTA Market-Frankford Line in Philadelphia. Westbound trains discharge passengers at the 69th Street station platform and go around the loop to one of two eastbound platforms at 69th Street to pick up passengers for their eastbound journey.
- Kennington tube station, on the London Underground Northern Line, trains from the Charing Cross branch can terminate at Kennington and then run around a loop to return north. Due to the layout, trains from the Bank branch may not access this loop.
- Schwabstraße Stuttgart S-Bahn, Loop is south of the station and completely underground
- Non-passenger loops: Porte Dauphine (line 2), Porte de Lilas (line 3bis), Porte de Clignancourt and Porte d'Orléans (line 4)
- Passenger loops: Nation (line 2), Charles de Gaulle-Etoile and Nation (line 6), Pré-Saint-Gervais (line 7bis)
- The western end of line 10 deserves a special mention, as it is long loop: trains arriving at Mirabeau from Gare d'Austerlitz enter a loop with a few stations on it: Eglise d'Auteuil, Auteuil, Porte d'Auteuil (official terminus), Michel-Ange Molitor, Chardon Lagache and Mirabeau again, to continue eastwards
- There also are a few loops used for stabling trains, for example west of Invalides and north of Porte de la Villette.
Balloon loops are used extensively on tramway systems that use single directional trams. Usually located at terminal stations, such a loop is actually a single one-way track round a block. Single directional trams have a cab at only one end and doors on one side, making them cheaper and having more space for passengers. On tram systems with bidirectional trams, balloon loops are not required.
The Milan interurban tramway network, although running bidirectional trams, sported balloon loops for termini within the city limits, so that those termini could be used as backup termini by the single directional trams used on urban service. In Milan, tramway depots are built as balloon loops.
New South Wales, Australia:
Both the French and the British terminals of the Eurotunnel Shuttle
service through the Channel Tunnel
consist of balloon loops.
Occasionally, balloon loops are used for reversing trains on lines with heavy grades and tight curves to equalise wear on both sides of locomotives and rollingstock. Such a balloon loop was constructed at Beech Forest on the 2ft 6in (762mm) VR line from Colac to Crowes.
The advantages of a balloon loop include:
- smooth operation.
- trains can arrive in any free platform, while another train is leaving any platform.
Compared to stations with stub platforms, balloon loops allow:
- fewer tracks and platforms would be required.
- arrivals into some platforms do not block departures from other platforms.
- time is not lost while drivers change ends and reset the train for the other direction.
- if the driver changed ends and discovers a hidden fault, then delays to trains are less likely.
The major disadvantage of a balloon loop is that it needs a lot of space to build, and even so, the curves can be very sharp. The very sharp 180 m radius curves on the Olympic Park
balloon loop cause noise, wear and tear on both the wheels and rails. Any platforms should be located on straight track, since if they are located on curved track, the gap between platform and carriage door can be a hazard.