Brian wrote:
GWR used to issue a green card that permitted approved botanising along
railway land. I can only remember that we were warned not to stand between
the rails when trains were coming! Very few cards were ever issued. Many
plants grew there that were missing or rare elsewhere.

Brian,

Can't help with motorways, but can advise railways.

DO NOT TRESPASS ON THE RAILWAY - you will be prosecuted or killed or
seriously injured. You need specialist training to go on or near the
railway, and need various safety systems & people in place.

Certainly LUL are quite aware of the ecological value of the
Underground, and might well have a person you could talk to about it.
Ring in to LUL's main sw/brd and ask. Alternatively I might be able to
advise a contact if you email me off group but it will take me a week or
two to find out for you.

Not sure about Network Rail, but they again might well have someone you
could talk to.

A better, easier bet would be to talk to any preserved railway societys
near you - probably much easier to get hold of the right person and gain
supervised access.

Disused railways might also prove of interest.

HTH,

Sarah