[John H-T]

When labour was cheap the grass etc. on the sides of the railway used be kept short to cut down fire risk. At the end of steam this was no longer necessary so the practice ceased, but slowly as Shed Cat suggests!

Photos from the age of steam very often show a lot fewer trees. I'm sure it not that simple but the end of steam did allow more trees to grow which is why preserved railways have to cut down so many trees!