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 2 ft 6 in (762 mm) Victorian Railways line from Colac to Crowes.
Advantages of a balloon loop include:
Compared to stations with stub platforms, balloon loops allow:
Disadvantages of a balloon loop include: