I've just stumbled over this (as if it's a fallen tree)
from Rail Engineer, June 2017:
Seeing the wood in the trees - Network Rail tackles troublesome trees
National survey
Network Rail decided that in order to alleviate this problem it would need to conduct a census of its estimated 10 million trees. With 20,000 miles of lineside to cover, work started in 2014 on what was essentially an aerial survey that made use of LiDAR (Light Detection and Ranging) mapping. This remote sensing method uses 3D laser scanning technology to measure the position of objects relative to the laser source.
Assessing risk
Surprisingly, trees that have fallen onto railway lines have rarely been old and rotten. Usually, they are healthy trees that have become vulnerable to wind forces. Factors that affect the stability of trees include the ways in which they are aligned or grouped. Isolated trees or those growing on steep slopes would be deemed of higher risk than those forming a group on the flat. Worst of all would be an isolated tree at the top of a rock cutting. Other higher risk trees include those growing adjacent to stations, near over-bridges, tunnel portals or power lines.
Size is important too, of course, with trunks greater than 150 mm in diameter being regarded as a potential threat should they fall across the track. Less than this and the damage they could potentially cause is not deemed a significant safety risk. The survey has been able to deduce the size of potentially vulnerable trees from their height and thereby assess the threat level they present. In total, approximately 100 different tree attributes have been included within the LiDAR survey analysis software.
Paul Meads, Network Rail’s head of lineside, said: “Our use of the tree census is emerging. Feedback will be useful for us to evaluate the benefits it can bring and we will be increasing our understanding of how it can best be used. Just now, the usage of the tree census is very much in its early days, but it promises to bring a real benefit to the rail industry as a whole.”
So far this year, Network Rail targeted and managed ‘hotspot’ areas for vegetation management covering 900 hectares, which accounts for roughly two per cent of the network. The work will continue and will include a reduction in the tree canopy in order to reduce leaf fall on the tracks. There will also be reduction in the lineside tree species that create the worst effect on rail adhesion.
It's a long article - even that's only a short excerpt. But basically, they now have a whizzy computerised map (part of their ORBIS project) of all neighbouring trees and will be conducting detailed interviews (of the kind that needs a chainsaw to hand) with some of them.
Looking at Chris's favourite picture of the tree that wanted to be a train driver (above), you might doubt that statement about "Less than this [150 mm] and the damage they could potentially cause is not deemed a significant safety risk."
PS: Sorry about the missing link, which I've now added.