According to website www.ukmotorwayarchive.org, construction of the Penrith By-pass, which went under BR's West Coast main line (WCML), started in November 1966 and opened to traffic two years later. It was not connected to the M6 motorway until October 1970, becoming the section between junctions 40 and 41.
Three bridges were designed by British Rail, two to carry the WCML over the motorway and one for the new westerly section of the A66 which passed under the railway. The bridge nearest Penrith station also carried the single line branch to Keswick. The superstructures were of post-tensioned in-situ concrete, each built alongside the railway track and traversed into position using closed circuit television as a means of controlling the operation.
The following historic photos - which are all my Copyright - of this challenging construction project date from April to June 1967 when work was underway on the by-pass, with BR steam used on related engineering trains as well as regular services in the area. With a project of this magnitude, the West Coast line had to be closed at times such as on the weekend of 3 - 5 June with trains diverted via the Settle & Carlisle route.
By the time the Penrith By-pass opened in November 1968 BR steam locos had vanished from the scene.
I have no detailed personal knowledge of the area, so observations about the exact locations are welcome.
The 45 ton oil-fired steam crane 1085/45 featured above has an interesting history. It was ordered in 1943 for the War Department, from Cowans,Sheldon of Carlisle, but actually manufactured by Ransomes and Rapier, Ipswich. In 1960 it was sold to British Railways London Midland Region. Converted from steam to diesel hydraulic in 1986, it became part of the Western Region's Plymouth breakdown train and is preserved on the Llangollen Railway.