Loop line has several meanings.
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 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.
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 the 2009 completion of the extension to Hammersmith.
