flat rail

A flat rail is a wrought iron or steel bar, typically rectangular in cross-section, laid flat on sleepers or the ground to form a track. Unlike the flanged rails that became standard in the 19th century, a flat rail has no vertical lip to guide wheels; instead, wheels or rollers are guided by a separate flange or by the edges of the bar itself. The rail itself carries the load horizontally and is held in place by direct fastening to wooden or metal sleepers.

Flat rails were the dominant form in early railways from the 1770s through the early 1800s. They typically measured 3 to 4 inches (around 75 to 100 mm) wide and 0.75 to 1.25 inches (roughly 20 to 30 mm) thick. Because they presented a low profile and were cheaper to produce than cast iron edge rails, they were widely adopted in mineral railways and industrial tramways. The weight per yard ranged from 8 to 16 pounds, suitable only for modest loads at low speed.

Wear and structural limits

The chief weakness of flat rails is poor load distribution. The narrow contact area between wheel and bar concentrates stress, causing rapid deformation and surface spalling. A loaded wagon rolling along flat rails could wear through the surface in months under regular use. Moreover, without a flange integral to the rail, the system depends entirely on the wheel flange to stay on track; any spread of the track gauge would derail wagons immediately. These limitations eventually made flat rails obsolete for main lines.

Flat rails persisted longer in quarries, collieries and works railways where traffic was intermittent or loads light. Some installations used a plate rail instead, which was an intermediate form: a flat bar with a slight raised edge cast into one side. Flat rails remain in use in specialist contexts, such as fixed-rail guides in machinery or short industrial sidings, but they have been supplanted by modern flanged rails for virtually all transport applications.