If you’re interested in the civil engineering aspect of railways there is a wealth of material out there about the construction process. Some of this has been captured on film. Going back to the 1950’s & 60’s there were British Transport films covering the regular maintenance work such as “Making Tracks” and moving into the 1970’s modernisation came into focus, with “Wires over the Border” showing the electrification of the West Coast Main Line, and “Operation London Bridge”, which gave an insight into the redevelopment of one of London’s major stations.
All these and many more are available on Youtube:
Moving onto the present day, Network Rail has been busy with time lapse video on some of its renewal projects, which show a speeded up version of events. A great example, which gives a great insight into modern construction methods is the one of Carmuirs Twin Tunnels, where the rail tunnels under the Forth & Clyde Canal were demolished and rebuilt to accomodate the wires for the electrification of the Edinburgh-Glasgow line.