the contacts which had been discovered in existing systems. The Strowger-Hudd system of Alfred Ernest Hudd (1958) used a pair of magnets, one a permanent magnet and one an electro-magnet, acting in sequence as the train passed over them. Hudd patented his invention and offered it for development to the Automatic Telephone Manufacturing Company of Liverpool (a subsidiary of the Strowger Automatic Telephone Exchange Company of Chicago). It was tested by the Southern Railway, London & North Eastern Railway and the London, Midland & Scottish Railway but these trials came to nothing. In 1948 Hudd, now working for the LMS, equipped the London, Tilbury and Southend line, a division of the LMS, with his system. It was successful and British Railways developed the mechanism further by providing a visual indication in the cab of the aspect of the last signal passed. In 1956, the Ministry of Transport evaluated the GWR, LTS and BR systems and selected the one developed by BR as standard for Britain's railways. This was in response to the Harrow & Wealdstone accident in 1952. Network Rail Network Rail (NR) AWS consists of: A permanent magnet set centrally between the rails and usually positioned such that it is encountered before the signal to which it relates. The top of the magnet casing is nominally level with the running surface of the rails (to within ). An electromagnet between the rails (with opposite polarity to the permanent magnet) positioned after the permanent magnet. Again the top of the casing is nominally level with the running surface of the rails (to within ). A cab indicator that can show a black disk or a yellow and black "exploding" disk, known as the "AWS sunflower" A control unit that connects the system to the brakes on the train A driver's AWS acknowledgement button An AWS control panel The system works on a set/reset principle. When the signal is at 'clear' or green ("off"), the electromagnet is energised. As the train passes, the permanent magnet sets the system. A short time later, as the train moves forward, the electromagnet resets the system. Once so reset, a bell is sounded (a chime on newer stock) and the indicator is set to all black if it is not already so. No acknowledgement is required from the driver. The system must be reset within one second of being set, otherwise it behaves as for