 The upper two lights are a two-aspect stop signal displaying either a green or red aspect, the lower two lights a repeater signal for the next stop signal ahead showing a green or yellow aspect, or no aspect when the stop signal shows a red aspect.Thus they are effectively four-light three-aspect signals with green/red/green/yellow lights from the top, danger being a single red light, caution the top green light over a yellow light, and clear two green lights.The Metropolitan line's suburban rail character opposed to other London Underground lines is also shown by the long distances between stops.