Green, Yellow, Red.
A Coney Island-bound (F) train led by R160A no. 9243 (Alstom, 2005-2010) passes a trio of colored flags at Avenue U station on the Culver Line. The three colors give different indications to train operators regarding the work zone south of here:
Red indicates the limits of the General Order (GO) on the southbound local track, which in this case has the track completely out of service. This is accompanied by a portable trip stop to prevent any train from accidentally entering the work limits.
The two yellow flags are an advance warning to southbound trains on the middle track, letting the operator know that there is a work zone on an adjacent track.
Green lets northbound trains on the middle track (since it is bi-directional) know that the entire train is clear of the work zone, and they can resume normal speed.
