en Cuevana 3. A film about one of the most responsible and professional jobs on British Railways. Practical work in shop and signal box, on gantry and trackside, coupled with instruction in mechanics, electricity, electronics and draughtsmanship, lead the apprentice intro the intricacies of design, the excitement of research and experiment, and the intense satisfaction of being in on a big changeover from old-style semaphore signalling to a new coloured light system.