Triple track is used in some parts of the New York City Subway, mostly in the Bronx, to add supplemental rush-hour services. The center track, which serves express trains, is signalled in both directions to allow two tracks to be used in the peak direction during rush hours; the outer tracks use bi-directional running and serve local trains exclusively in one direction. During service disruptions on one of the two outer tracks, trains could also bypass the affected sections on the center track.