Balloon loop

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.

Advantages of a balloon loop include:

• Smooth operation
• Trains can arrive and leave simultaneously, as long as an open platform exists.
• Reversal allows even wear and tear on the wheels.
• Eliminates the need for brake test if locomotives uncoupled to carry out run around the move.

Compared to stations with stub platforms, balloon loops allow:

• Fewer tracks and platforms are 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, delays are less likely.

Disadvantages of a balloon loop include:

• Very space-consuming.
• The sharp curves cause noise, as well as wear on wheels and rails.
• If the platform is located on the curve, the gap between the platform and railcar door is a hazard.
• 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 balloon 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.

View of the balloon loop at the temporary PATH World Trade Center station from above.

• 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.[1]
• 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.