Balloon loop

A balloon loop, turning loop or reversing loop (North American) allows a rail vehicle or train to reverse direction without having to shunt or even stop. Balloon loops can be useful for passenger trains and unit freight trains such as coal trains.
Balloon loops are common on tram or streetcar systems. Many streetcar and tram systems use single-ended vehicles that have doors on only one side and controls at only one end, or they haul trailers with no controls in the rear car and so must be turned at each end of the route.
History.
Balloon loops were first introduced on tram and, later, metro lines. They did not commonly appear on freight railways until the 1960s, when the modernising British Rail system introduced "merry-go-round" (MGR) coal trains that operated from mines to power stations and back again without shunting.
Tramways.
Balloon loops enable higher line capacity (faster turnaround of a larger number of trams) and allow the use of single-ended trams which have several advantages, including lower cost and more seating when doors are on one side only. However, double-ended trams also benefit from the capacity advantage of balloon loops, for example on the former Sydney tram system where loops were used from 1881 until the second-generation system's closure in 1961. Initially the Sydney system was operated by single-ended steam trams and then, from the 1890s, by double-ended electric trams. Lines were looped in the Sydney CBD and the other busiest areas of operation, such as the eastern suburbs lines, as they provided greater turn-around capacity on this very busy system. The Sydney system was the first example of a tramway system using loops and has continued to build them up to 1997 (third-generation system). 
Later in the 19th century, looped streetcar (tram) lines also began to appear on systems in the US and soon looped operation with single-ended streetcars became widely used on many North American streetcar systems. European systems were almost universally converted to looped operation in the early 20th century, and most also adopted single-ended trams. Loops were also used on some tramways in Asia, South America and New Zealand, as well as on some other Australian systems in addition to Sydney. Looped operation with single-ended trams is still the predominant method of tramway operation in the world, in spite of the recent construction of some new, typically smaller, stubbed systems with double-ended trams.
Examples.
Passenger.
On a balloon loop, the station is on the balloon loop, and the platform may be either curved or straight.
Austria.
The tram systems in Vienna, Graz, Linz and Innsbruck employ Balloon Loops
Other.
Multiple stations on a balloon loop:
With balloon loop:
The balloon loop is past the station.
Tram Systems.
Balloon loops are used extensively on tramway systems with single-ended trams. Usually located at termini, the loop may be a single one-way track round a block. Single-ended 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 double-ended trams balloon loops are not required but may still be used as they can provide greater turn-around capacity than a stub terminus; the Birmingham Corporation Tramways terminus at Rednal had a balloon loop in addition to the conventional stub tracks, providing extra capacity to handle weekend and bank holiday crowds visiting the nearby Lickey Hills. 
The Milan interurban tramway network, although using double-ended trams, had balloon loops at termini within the city limits so that they could be used as backup termini by the single directional trams used on urban service. In Milan, tramway depots are built as balloon loops, just as urban termini. 
Another example is in Potsdam, Germany.
Freight.
Australia.
Loading loops
Unloading loops
Iran.
Has several balloon loops and triangles.
United Kingdom.
There are several balloon loops at power stations in the UK; these have been provided so that coal trains may unload without stopping (known as the "merry-go-round" system). Examples include Cottam, Didcot, Drax, Eggborough, Ferrybridge, and Ratcliffe-on-Soar.
Also, the Fife Circle line between Edinburgh and the county of Fife acts like a giant balloon loop, branching off after Inverkeithing and connecting again at Kirkcaldy.
Also, two London Underground lines have balloon loops; the Northern Line has one at Kennington, where trains can terminate, drive around the reversing loop, and then start again whilst others can pass through; whereas the Piccadilly Line has one serving Terminal 4 of Heathrow Airport (half of all trains use this loop to return eastwards back into London, while trains terminating at Terminal 5 must halt and reverse.
Other.
Both the French and the British terminals of the Eurotunnel Shuttle service through the Channel Tunnel consist of balloon loops, in opposite directions to even out wear on the wheels.
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 Victorian Railways line from Colac to Crowes.
Advantages.
Advantages of a balloon loop include:
Compared to stations with stub platforms, balloon loops allow:
Disadvantages.
The major disadvantage is that a balloon loop is very space consuming. Another disadvantage is that the sharp curves cause noise, as well as wear on wheels and rails. Also, if the platform is located on the curve, the gap between the platform and railcar door is a hazard. The former South Ferry station on the New York City Subway solved this problem by using gap fillers that extended out to the railcar door when the train triggered a switch on the tracks. The older station had been closed, but was reopened as a result of damage to the newer station caused by Hurricane Sandy.
On systems where, for reasons of economy, the couplings are made non-reversible (e.g. by fitting the air brake pipe along one side of the car only), the use of a reversing loop will cause a proportion of the rolling stock to face the "wrong" way and it may not be possible to assemble a complete train in a depot, even if sufficient cars are on hand. This was the case on the Charing Cross, Euston and Hampstead Railway (now part of the Northern line of London Underground). After the opening of a loop at Charing Cross (Embankment) in 1914 (replaced in 1926 by the present Kennington loop) car ends were marked "A" or "B" (later, when axles were designated by letters, the "B" car ends became "D" to match the adjacent axle), and it was not permitted to couple cars together if the ends to be coupled bore the same letter. It was found necessary to provide a turntable at Golders Green depot (near the other end of the line), for use when there was an imbalance of car directions.
To avoid this problem, on many systems with a balloon loop the couplings and brake hoses are made reversible.
At coal ports such as Kooragang in Newcastle, New South Wales the space inside the balloon loops is used for storing coal, so that it is not wasted.
At the Olympic Park station in Sydney, the loop is flattened where the platforms are located, so that the platform faces are straight.