> If the rail was newly laid in the 50s, then they might have had to cut down the trees
It wasn't. The first London to Brighton railway line opened in 1841. Most of sections of the two main paths from Brighton to London (to London Victoria and London Bridge respectively) should have been complete by about 1855. There was expansion work (adding extra tracks) on the busiest section of the line around 1908, but I think that was the last major work on the London-Brighton lines apart from connecting branches.
There's of course been replacements and upgrades, but these tends to happen bit by bit.
It was after the war, and the US was doing enormous infrastructure projects, so it seemed plausible that they did something around that time. Plus, they made a film to show it. But it was really hard to see the trees in the film, so it could be an illusion.
It wasn't. The first London to Brighton railway line opened in 1841. Most of sections of the two main paths from Brighton to London (to London Victoria and London Bridge respectively) should have been complete by about 1855. There was expansion work (adding extra tracks) on the busiest section of the line around 1908, but I think that was the last major work on the London-Brighton lines apart from connecting branches.
There's of course been replacements and upgrades, but these tends to happen bit by bit.