Yes, this does my head in too. on my journey to Waterloo East from High Brooms every day I can see Buddleia in between the bricks in overbridges, in retaining walls, in viaducts, actually all over the roof of Hither Green station buildings and covered (out of use) underpass, and just generally in the ballast.
And I can see the brick layers being distorted, bricks coming out of place etc.
All in all a great danger to the railway (some bridge parapets are going to fall on the track) and storing up a huge cost.
As someone from a TOC recently said to me: "if you had Buddleia growing out of the walls or roof of your house, you'd do something about it. Why doesn't Network Rail?"
I too frequently pass through Hither Green on the Sidcup line and the buddleia bushes at the end of platform 5 are enormous and must have been there for years. If you travel along the down line from Cannon Street towards New Cross / Greenwich there is a profusion of buddleia growing alongside the track, despite much of that track having been replaced in the last year or two. A journey today along the South London line from Clapham High St towards Denmark Hill shows enormous buddleia bushes sprouting from the viaduct. I noticed trees growing next to the tracks with roots several inches wide. And I vividly remember the tree inbetween the Charing Cross and Cannon Street lines near St Johns which remained in place notwithstanding replacement of track a few feet away, it may still be there. For the last year the fast lines to Charing Cross along the viaduct which leads into the new Bermondsey diveunder tracks have been closed and engineering staff are often on site working on the tracks, installing new signals and cabling, and repairing portions of the viaduct brickwork. Despite this and the lengthy closure, buddleia still festoons the viaduct's brickwork.
I fully appreciate that resources are tight and that vegetation management has to compete with other priorities. Removing buddleia growing on the outside surface of viaducts in particular must be a real challenger given the difficulties in access. But surely some attempts could be made during routine line possessions to at least trim the plants if they can't be completely removed, all that is needed is a pair of secateurs and a saw ?