What is line clearance? : Describes the distance between two load-bearing train tracks forming an railway line. The percentage of railroad tracks in the world uses 1.435mm (4½ inches) as standard line clearance. Line openings at different distances are called wide or narrow. The line is interrupted where two different lines of lines meet. For this reason, a number of lines were constructed in order to be able to switch between the different line widths in some transition zones.
The only type of rail without a line opening is the line structure called monorail. Some light-load electric railway lines may have a third or fourth line. This line located outside the main line is used for feeding the electricity grid.