Though the subway system operates on a 24-hour basis, during late night hours some of the designated routes do not run, run as a shorter route (often referred to as the 'shuttle train' version of its full-length counterpart) or run with a different stopping pattern. These are usually indicated by smaller, secondary route signage on station platforms. Because there is no nightly system shutdown for maintenance, tracks and stations must be maintained while the system is operating. This work sometimes necessitates service changes during midday, overnight hours, and weekends.