Рельстік көліктің айналу тізбегі

Rail loop for turning vehicles

A balloon loop, turning loop, or reversing loop (North American Terminology) allows a rail vehicle or train to reverse direction without having to shunt or stop. Balloon loops can be useful for passenger trains and unit freight trains. Balloon loops are common on tram and streetcar systems. Many streetcar and tram systems use single ended vehicles that have doors on only one side and controls at only one end. These systems may also haul trailers with no controls in the rear car, and, as such, must be turned at each end of the route.

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.

On the former Sydney tram system, 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; 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 the crowded 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 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.

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. 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. However, double ended trams also benefit from the capacity advantage of balloon loops. Occasionally, balloon loops are used for reversing trains on lines with heavy grades and tight curves to equalize wear on both sides of locomotives and rollingstock. Such a balloon loop was constructed at Beech Forest on the 2ft6in Victorian Railways line from Colac to Crowes.

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 Olympic Park, the loop is flattened where the platforms are located, so that the platform faces are straight. 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. The former South Ferry station on the New York City Subway solved the railcar door hazard by using gap fillers that extended out to the railcar door when the train triggered a switch on the tracks. The older station was closed in 2009, but was reopened between 2013 and 2017 as a result of damage to the newer station caused by Hurricane Sandy.