This is a repeater signal. Repeaters always have a black 'R' on a white circular board fixed below it. Note that the starter signal for the platform is not visible from here. This is why the repeater is needed. It will always stay at yellow as long as the starter is red. When the starter is taken off (goes yellow), the repeater turns green. Nagpur Station, Platform No.7, 2005-08-16. Alok Patel.