The Automatic Warning System (AWS) was introduced in the 1950s in the United Kingdom to provide a train driver with an audible warning and visual reminder that they were approaching a distant signal at caution. Its operation was later extended to give warnings for; A colour light signal displaying a double yellow (steady or flashing), single yellow or red aspect A reduction in permissible speed A temporary or emergency speed restriction An automatic barrier crossing locally monitored (ABCL), an automatic open crossing locally monitored (AOCL), or an open crossing (OC). AWS was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system. An earlier contact system, installed on the Great Western Railway since 1906 and known as automatic train control (ATC), was gradually supplanted by AWS within the Western Region of British Railways. Principles of operation Information is conveyed by magnetic fields to the moving train through equipment fixed in the middle of the track, known as an AWS magnet. The system works by the train detecting sequences and polarities of magnetic fields passing between the track equipment and the train equipment via a receiver under the train. Running over the permanent magnet of an AWS magnet with its south pole facing upward arms the system; subsequently running over an active electromagnet of the AWS magnet with its north pole facing upwards disarms the system. The electromagnet is connected to the green signal aspect so the AWS receiver will only be disarmed if the signal is displaying green; any other aspect leaves the AWS system armed. Multiple unit trains have a receiver at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one; this could be either at the front or rear depending on the direction the vehicle is travelling in. The equipment on a train consists of; A magnetically operated 'receiver' underneath the front of the train A relay control box A visual indicator (known colloquially as a 'sunflower') An audible sounder which produces either a 'horn' or 'bell' sound An AWS/TPWS acknowledgement button An AWS/TPWS Driver Machine Interface (indicator panel) An interface with the train's emergency braking system A static voltage converter providing operating voltages of 12 V and 40 V from the unit supply An isolating switch to disconnect the AWS for maintenance or if the AWS develops a fault in service AWS at signals As