A transition curve is a spiral-shaped length of highway or railroad track that is used between sections having different profiles and radii, such as between straightaways (tangents) and curves, or between two different curves.
This sign aside a railroad (between Ghent and Bruges) indicates the start of the transition curve. A parabolic curve (POB) is used.
Brusio spiral viaduct and railway (Switzerland, built 1908), from above
A railway track or railroad track, also known as a train track or permanent way, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties and ballast, plus the underlying subgrade. It enables trains to move by providing a dependable surface for their wheels to roll upon. Early tracks were constructed with wooden or cast iron rails, and wooden or stone sleepers; since the 1870s, rails have almost universally been made from steel.
New railway concrete sleeper
Traditional railway track showing ballast, part of sleeper and fixing mechanisms
Track of Singapore LRT
Ballastless high-speed track in China