A mountain railway is a railway that ascends and descends a mountainslope that has a steep grade. Such railways can use a number of different technologies to overcome the steepness of the grade. Mountain railways commonly have a narrow gauge to allow for tight curves in the track and reduce tunnel size and structure gauge, and hence construction cost and effort.
On steep grades the friction between the wheels and the rails cannot apply sufficient adhesion to the train's wheels so as to overcome gravity, and the train is in danger of sliding down the track. In practice this affects downhill braking capability before it affects uphill climbing ability, and some mountain railways at the lower end of the steepness spectrum rely on standard adhesion for propulsion, but use special track brakes acting directly on the rails.
Where the line is too steep to rely on adhesion for climbing, a rack railway may be used, in which a toothed cog wheel engages with a toothed rack rail laid between the tracks. A now little used alternative to the rack and pinion railway is the Fell system, in which traction and/or braking wheel are applied to a central rail under pressure.
Finally at the steepest end of the spectrum, a funicular railway may be used. Here a cable is used to haul counterbalenced trains up and down the track. The car itself is often custom built for the slope, with specially raked seating and steps rather than a sloped floor. Taken to its logical conclusion as the slope becomes vertical, a funicular becomes an elevator (British English: lift).
