Domestic weedkillers typically contain 1% glyphosate (1g/l), whereas commercial preparations go up to 49%. (I wonder what NR uses?) As option says, it only works on actively growing plants as it needs to be absorbed by the plant.
There's an interesting piece (
here) about glyphosate on NR's site, describing its efforts at reducing the amount of herbicides used in vegetation management - as well as a link to the Varley review, which looked at balancing the needs of running the railway safely against the wildlife value. There's data on vegetation strikes, and how much is being spent over time - showing clearly how little was spent in 2013/14.