At this time the GWR was developing its automatic train control (ATC) system. When finalised, track equipment was provided at distant signals which gave an audible and visual indication in the engine cab of whether the distant signal was off or on. If it was on, and the driver failed to acknowledge a warning, the brakes were automatically applied. In the experimental stage on the Lambourn branch, the ATC was provided "instead" of distant signals; conceptually an early form of cab signalling.