It's almost 40 years since steam disappeared from widespread use on the railways, but the development of steam locomotives has never stopped. Technical advances in fuel, efficiency, design and new materials have been adopted and this "modern steam" is now pulling passengers and freight in several corners of the world. Claire Barratt meets some of the dedicated engineers working with modernised locomotives and hears about a brand-new design on the drawing board for a 21st-century loco. Producer Alan Daulby