A secondary signal placed in advance of a main signal to give the crew early warning of its aspect. It is typically installed on sharp curves, in tunnels, or where structures block the line of sight. The repeater itself does not control the block, but allows the engineer to adjust speed before the actual signal comes into view.
Heard on the job
You need to start braking as soon as you see the banner repeater drop, because the home signal is completely blind around that rock cut.