Night Train said:When I heard this I thought it was a good thing, London through Manchester. Then I thought about the objections to every bit of any proposed route that it may take due to NIMBY attitude.
Then I thought M1 M6. Why are train lines not run with major motorways as a double deck where ever possible and adjacent where not? Road on top, rail below, rail on a bridge or in a cutting and the road adjacent following the contour of the land. If that was designed at the very start then it would be easy.
The 2nd to last major train line built was in 1899, so 'BC' (Before car). The next rail line to be built was over 100 years later 2007, HS1, the Eurostar route from London to the tunnel.
It mostly runs alongside the M2 through Kent, and crosses rivers like the Medway on joint road/rail bridges, so in answer to the question of "why not" is "they do!"