Brambles and nettles through chain link fence

My garden backs on to a railway line in a cutting, with roughly 4m/12ft of level Network Rail ground before the cutting drops sharply. This ground is over-run with brambles and nettles that grow through Network Rail's 1.6m/5ft chain link fence. I have Forsythia on my side of the fence.
Trying to get Network Rail to control their weeds has been a waste of time. I know that just cutting back the brambles is counter-productive. I could try to dig up or use weedkiller on the brambles, but the area I'd have to treat would be at least 15m/50ft wide and I'm concerned about it quickly growing over again.
I could put up a solid (eg tongue-and-groove) fence, but I'm looking at over £1,000 even if I DIY. I thought of putting some matting impervious to brambles on the fence then some form of screen such as bamboo or willow screening in front, which should cost under £200. I can't, though, find a suitable matting.
Can someone please help me with some advice. Thank you in advance for any input.
Trying to get Network Rail to control their weeds has been a waste of time. I know that just cutting back the brambles is counter-productive. I could try to dig up or use weedkiller on the brambles, but the area I'd have to treat would be at least 15m/50ft wide and I'm concerned about it quickly growing over again.
I could put up a solid (eg tongue-and-groove) fence, but I'm looking at over £1,000 even if I DIY. I thought of putting some matting impervious to brambles on the fence then some form of screen such as bamboo or willow screening in front, which should cost under £200. I can't, though, find a suitable matting.
Can someone please help me with some advice. Thank you in advance for any input.
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I'm wondering (to myself) of the virtue of planting things dense shrubs / trees to shade out the weeds. I'm wondering you found some free, large elder saplings and planted them on the other side of the fence, whether it might help. Or plant on both sides. They say that brambles and nettles are a precursor to saplings and trees growing up. A load of elder (or something as easy come by and quick growing) might help. Hazel, hawthorn? Thick native hedging / shrubs would 1) darken the earth 2) take up water and 3) nutrients 4) create a dense root network. Just a thought.
I have a similar problem except the brambles climb a 5' stone wall, rooting as they go, heading for my garden. There is now a bank of brambles about 6' wide, about 100' in length, growing from my neighbour's field, which he does nothing with. I have finally resorted to spraying over the top of the bank with glyphosate which I buy in 5lt containers from the local farm suppliers. Much cheaper than small commercial brands. Unfortunately the first time I did it I managed to kill some of my own shrubs with spray drift. I have to wait for the wind to blow from the East but of course the wind eddies. The most effective time to spray is July, however, I think established brambles need spraying 4 times a year.
If you decide spraying is the path to go down you could possibly use a sheet of thin plywood held against the fence whilst spraying to protect plants on your side. Your forsythia should be fine as long as the spray does not get onto the leaves. All you can aim for is to create a clear area between the fence and your garden, and keep at it annually.
Good luck!