time, railways are asked to transport exceptional loads such as massive electrical transformers that are too tall, too wide or too heavy to operate normally. Special measures must be carefully taken to plan successful and safe operation of Out-of-gauge trains. For example, adjacent tracks of a double line might have to be shut down to avoid collisions with trains on those adjacent tracks. Passing lanes These are a form of crossing loop, but are so long as to allow trains approaching each other from opposite directions on single-track lines to cross (or pass) each other without reducing speed. In order for passing lanes to operate safely and effectively, trains must be timetabled so that they arrive at and enter the loop with close time tolerances, otherwise they will need to slow or even be brought to a complete stop to allow the oncoming train to pass. They are suited to lines with light to moderate traffic. An example of where passing lanes have been installed in order to improve travel times and increase line capacity is the section of the Main Southern railway line in Australia between Junee and Albury. This was built as a single track line in stages between 1878 and 1881, and was partially duplicated between 2005 and 2010 by the construction of four passing lanes each long. In this instance, this was accomplished by extending pre-existing crossing loops of either or in length. Construction Duplication The process of expanding a single track to double track is called duplication or doubling, unless the expansion is to restore what was previously double track, in which case it is called redoubling. The strongest evidence that a line was built as single-track and duplicated at a later date consists of major structures such as bridges and tunnels that are twinned. One example is the twin Slade tunnels on the Ilfracombe Branch Line in the UK. Twinned structures may be identical in appearance, or like some tunnels between Adelaide and Belair in South Australia, substantially different in appearance, being built to different structure gauges. Tunnel duplication Tunnels are confined spaces and are difficult to duplicate while trains keep on running. Generally they are duplicated by building a second tunnel. An exception is the Hoosac Tunnel, which was duplicated by enlarging the bore. Carried-out provision for duplication To reduce initial costs of a line that is certain to see heavy traffic in