A flashing single or double yellow aspect indicates that a train is to take a diverging route ahead with a lower line speed than the main route, indicating to the driver to slow the train down in time for the speed limit of the diverging route. A flashing double yellow (only used in 4-aspect territory) means that the next signal is showing flashing single yellow. A flashing single yellow means that the next signal at the junction is showing (steady) single yellow with an indication for a diverging route, and the signal after (in advance of) the junction may be red. When the train has neared the junction and slowed down, the junction signal may 'step up' to the correct aspect depending on the state of the line ahead. Flashing Yellows are generally provided where the diverging route's speed limit fairly high, and approach control from red would cause drivers to slow down too much.
Also see Approach control .
Last edited by GeoffM on 15/09/2016 at 03:00