Costing at the time 1.8 million to complete, the Severn Tunnel was a Victorian engineering triumph, and for over a century it remained the longest rail tunnel in Britain. Construction had begun in 1873 but came to a standstill after the workings were inundated by water in 1879. An accomplished civil engineering contractor, Thomas Andrew Walker (1828-89) had worked on railways in Canada, Russia and Africa as well as on London's new underground lines;...