Loop line (railway)

• A less important line which leaves a main line and then rejoins it later, continuing in the same direction. Significantly longer than a passing loop, its purpose may be purely to provide a bypassing route, or it may provide a goods or passenger service in its own right, such as the Hertford Loop line.
• A configuration, sometimes known as a balloon loop or horseshoe curve where trains entering it turn through a half circle and return to the start of the loop facing in the opposite direction from which they came, such as the Sutton Loop. The Darjeeling Himalayan Railway, a UNESCO World Heritage site railway in India has several examples of these in addition to six full zig zags and 3 complete spirals.
• A circle route is a totally enclosed system whereby trains always remain in the loop, as in the Glasgow Subway or the London Underground Circle line before it was extended to run over the Hammersmith branch in 2009.