The distance between the centre lines of two parallel tracks. Every track system has a standard value, reached with a turnout plus compensating curve or with two concentric curves.
On Märklin C track the track spacing is 77.5 mm: turnout 24611 with compensating curve 24224 ends exactly that far beside the main track. On K track it is 64.6 mm, on Minitrix 33.6 mm.
Märklin C track: articles, geometry and the track planner