Todya while waiting for the 15:49 train to Orpington, the signal at the end of platform 2 was displaying yellow (on down relief line), and while looking towards it (and Bromley South), noticed it change to double yellow, then a second later go back to yellow again. Why might this have happened? The signal stayed yellow for at least 5 mins, (red guards Bromley South in this case) and when the train arrived and progressed to Bromley South, did so as normal but the driver went slowly till a green before the station showed. My first thought was the bulb went on the second yellow light, which if true would have been a very rare catch. Given the signal beyond was green by the time my train got there suggests no train was held at Bromley South, and if there was the signal should not have tried to go double yellow. There are no junctions enabling crossover between main and relief lines between the Shortlands and Bickley platforms, the crossovers being either side. Any thoughts?