Black Bakelite track circuit repeaters, designed to indicate the location of a train at a specified (track circuited) section. A low electrical current passed through the rails is short-circuited by the train’s wheels, thus trigering the repeaters indication to change from 'track clear' to 'track occupied'. Located on model railway layout, gauge 1, Lancashire & Yorkshire Railway Signalling School, built 1912, by L&YR and Bassett-Lowke Ltd, with associated equipment. Lancashire & Yorkshire Railway Signalling School Model Railway Layout 1912