The Fell system was the first third-rail system for railways that were too steep to be worked by adhesion on the two running rails alone. It uses a raised centre rail between the two running rails to provide extra traction and braking, or braking alone. Trains are propelled by wheels or braked by shoes pressed horizontally onto the centre rail, as well as by the normal running wheels. Extra brake shoes are fitted to specially designed or adapted Fell locomotives and brake vans, and for traction the locomotive has an auxiliary engine powering horizontal wheels which clamp onto the third rail. The Fell system was developed in the 1860s and was soon superseded by various types of rack railway for new lines, but some Fell systems remained in use into the 1960s. The Snaefell Mountain Railway still uses the Fell system for (emergency) braking, but not for traction.
The Fell system was designed, developed and patented by British engineer John Barraclough Fell. The first test application was alongside the Cromford and High Peak Railway's cable-hauled incline at Whaley Bridge in Derbyshire, England, in 1863 and 1864.
These tests attracted the attention of the French Government, which conducted its own tests on the slopes of Mont Cenis in 1865. As a result, the Mont Cenis Pass Railway was built as a temporary connection between France and Italy whilst the tunnel under the Alpine pass was being built.
The following railways have used the Fell system. Of these, the only one still in operation is the electrified Snaefell Mountain Railway on the Isle of Man, which occasionally uses the centre rail for braking only; the cars are all now equipped with rheostatic braking, which meets all normal braking needs. The only surviving Fell locomotive, New Zealand Railways H 199, is preserved at the Fell Locomotive Museum, Featherston, New Zealand, near the site of the Rimutaka Incline.