Minimum railway curve radius

The minimum railway curve radius is the shortest allowable design radius for railway tracks under a particular set of conditions. It has an important bearing on constructions costs and operating costs and, in combination with superelevation (difference in elevation of the two rails) in the case of train tracks, determines the maximum safe speed of a curve. Minimum radius of curve is one parameter in the design of railway vehicles as well as trams.Monorails and guideways are also subject to minimum radii.

The first proper railway was the Liverpool and Manchester Railway which opened in 1830. Like the trams that had preceded it over a hundred years, the L&M had gentle curves and gradients. Among other reasons for the gentle curves were the lack of strength of the track, which might have overturned if the curves were too sharp causing derailments. There was no signalling at this time, so drivers had to be able to see ahead to avoid collisions with previous trains. The gentler the curves, the longer the visibility. The earliest rails were made in short lengths of wrought iron, which does not bend like later steel rails introduced in the 1850s.

Minimum curve radii for railroads are governed by the speed operated and by the mechanical ability of the rolling stock to adjust to the curvature. In North America, equipment for unlimited interchange between railroad companies are built to accommodate 350-foot (106.7 m) radius, but normally 410-foot (125.0 m) radius is used as a minimum, as some freight cars are handled by special agreement between railroads that cannot take the sharper curvature. For handling of long freight trains, a minimum 717-foot (218.5 m) radius is preferred.

The sharpest curves tend to be on the narrowest of narrow gauge railways, where almost everything is proportionately smaller.

As the need for more powerful (steam) locomotives grew, the need for more driving wheels on a longer, fixed wheelbase grew too. But long wheel bases are unfriendly to sharp curves. Various types of articulated locomotives (e.g. Mallet, Garratt and Shay) were devised to avoid having to operate multiple locomotives with multiple crews.

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