Parts of the track on which engineering work is being carried out are generally indicated by the following signals: a track circuit operating device is applied to the track to turn the signal to danger; three detonators are placed at either end, spaced 20 yards apart; a red banner flag or board is placed by the middle detonator; at night, a steady or flashing red light is used; marker boards with two vertical flashing lights are placed by the worksite (red lights for the entrance and yellow for the exit)/