Ordinary railway lines usually look as though they are laid on level ground. In reality most stretches of railway track have a slight gradient, so that during a journey of any reasonable length, a train will be running up and down a number of very gradual slopes. However, there is definite limit to the degree of steepness a train can manage going up hills before its wheels start to skid on the tracks. In mountainous areas especially, it is not always possible to avoid gradients which a normal train simply could not climb, even by carrying the track through deep cuttings and tunnels. To overcome this difficulty rack railways have been built. Locomotives intended to run over such sections of track have a toothed, or cog, wheel fitted between the normal running wheels. This connects with a third rail, also toothed, fitted to the track itself. This is the rack rail. When the locomotive runs along the track, its cog wheel engages the rack rail and so holds the train to the track with no risk of skidding.
Some rack railway lines pass through mountainous regions. Such as Alps. Others have been built to carry passengers to the actual summits of mountains, such as Snowdon in North Wales, and Mount Washington and Pike’s Peak inn the United States.