Rack railway systems use a geared wheel on the loco to engage with a rack, a toothed rail or rails set in the track, where the grade is too steep for the normal adhesion (friction) between wheels and rails to be used.
A 'rack and pinion' system usually refers to a pure rack system where all the track in the system is provided with a rack, including at points and crossovers. A 'rack and adhesion' system is one where not all of the track has a rack, and normal working using adhesion or friction between the rails and the driver wheels of the loco is used in some parts. The Nilgiri Mountain Railway is a rack and adhesion railway.
Among different kinds of rack systems are the 'Abt system' using a central rack rail or rails with two or more rows of teeth that are offset from one another, the 'Riggenbach' or 'Riggi' system where the rack is formed by short cross-bars attached to two parallel rails much like a ladder, and the 'Strub' system which has a single rack rail with teeth cut into it. The Nilgiri Mountain Railway was supposed to have the Riggenbach system at first, but for various reasons ended up with the Abt system for its rack.
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