several times the length of the trains using it, and is suitably signalled, then trains proceeding in opposite directions can pass (cross) each other without having to stop or even slow down. This greatly reduces the time lost by the first train to arrive at the crossing loop for the opposing train to go by. This system is referred to as a dynamic loop. Overlaps and catch points Some railways fit catch points at the ends of crossing loops so that if a train overruns the loop, it is derailed rather than collide with an opposing train. Since the available space for crossing loops is usually limited, they do not normally have an overlap (safety margin) between the starting signals and the end of the double line. In Australia, the Australian Rail Track Corporation (ARTC) policy provides for overlaps of about 500 m and 200 m respectively in an effort to avoid derailment or collision. Automatic operation Many crossing loops are designed to operate automatically in an unattended mode. Such loops may be track-circuited with home signals cleared by the approaching train. Some loops have the points in and out of the loop operated manually, albeit more recent examples have so-called self-restoring switches that allow trains to exit a loop without needing to change the points. Other forms of remote operation included centralized traffic control, in which a train controller changes points and signals from a remote office; and driver-operated points, which enable train crews to use a radio system to set the points from a distance. Gradients The design of crossing loops may have to be modified where there are severe gradients that make it difficult for a train to restart from a stationary position, or where the terrain is unsuitable for a normal loop. A crossing loop on steep gradient may have catch points on the downhill end to reduce the impact of runaways. Train length Since central operation of the points and signals from a single signal box is convenient, and since there are practical limits for the distance to these points and signals, crossing loops can have a system-wide effect on train sizes. Line capacity Line capacity is partly determined by the distance between individual crossing loops. Ideally these should be located at inverse-integer intervals along the track by travel time. The longest section between successive crossing loops will, like the weakest link in a chain,