Siding (rail) in the context of "Railroad switch"

The Alaska Railroad (reporting mark ARR) is a Class II railroad that operates freight and passenger trains in the state of Alaska. The railroad's mainline runs between Seward on the southern coast and Fairbanks, near the center of the state. It passes through Anchorage and Denali National Park, to which 17% of visitors travel by train.

The railroad has 656 miles (1,056 km) of track, including sidings, rail yards and branch lines. The main line between Seward and Fairbanks is over 470 miles (760 km) long. The branch to Whittier conveys freight railcars interchanged with the contiguous United States via rail barges sailing between the Port of Whittier and Harbor Island in Seattle.

The main line, or mainline in American English, of a railway is a track that is used for through trains or is the principal artery of the system from which branch lines, yards, sidings, and spurs are connected. It generally refers to a route between towns, as opposed to a route providing suburban or metro services. It may also be called a trunk line, for example the Grand Trunk Railway in Canada, or the Trunk Line in Norway.

For capacity reasons, main lines in many countries have at least a double track and often contain multiple parallel tracks. Main line tracks are typically operated at higher speeds than branch lines and are generally built and maintained to a higher standard than yards and branch lines. Main lines may also be operated under shared access by a number of railway companies, with sidings and branches operated by private companies or single railway companies.

A structure gauge, also called the minimum structure outline, is a diagram or physical structure that sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. It specifies the height and width of station platforms, tunnels and bridges, and the width of the doors that allow access to a warehouse from a rail siding. Specifications may include the minimum distance from rail vehicles to railway platforms, buildings, lineside electrical equipment cabinets, signalling equipment, third rails or supports for overhead lines.

A related but separate gauge is the loading gauge: a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. The difference between these two gauges is called the clearance. The specified amount of clearance makes allowance for wobbling of rail vehicles at speed or the shifting of vehicles on curves; consequently, in some circumstances a train may be permitted to go past a restricted clearance at very slow speed.