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Currently covering only Sydney, Hidden City claims to allow the user to "explore cities in a new and exciting way", and offers to take the user on a "mysterious and fun adventure that explores the secrets and wonders within your city".
Presumably, the wonders and secrets that the app refers to must be the workings of the CityRail timetable.
As one of the few apps to have access to CityRail's real-time data, perhaps it can tell us how that mysterious mechanism works.