You are misusing the term - in rail systems, double-tracked means that the system (or line) has two tracks, typically one for each direction. Their are systems

You are misusing the term - in rail systems, double-tracked means that the system (or line) has two tracks, typically one for each direction. Their are systems (MTA/NYCT most notably in the USA) that have more than two tracks, although in NY's case, it's to allow local trains and express trains to run on different tracks. In a single-track system 24/7 operation is not possible: In double-tracked systems, there are crossovers (typically located near stations, but they can be anywhere) or switches to storage tracks that allow traffic to be routed along single tracks as necessary to allow track segments to be taken out of service.