Network Rail is now subject to the Freedom of Information Act so you can make a formal FOI request for the information you want:

http://www.networkrail.co.uk/foi/

The following site might be useful as it lists FOI requests other people have made to NR (and other public organisations) and there might be something specific to Japanese knotweed. If you made your request via this site both your request and NR's response would be public (so potentially useful to other people concerned about JK on NR land) but it would mean your name (and address/locality as you'll need to put them in the request) would be publicly visible, so overall probably better to approach NR directly and in private.

https://www.whatdotheyknow.com/body/network_rail