Duplication

The process of expanding a single track to double track is called duplication or doubling.

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 line in the United Kingdom. Twinned structure may be identical in appearance, or like some tunnels between Adelaide and Belair South Australia, substantially different in appearance.