Has the Japanese Knotweed encroached onto your property/land ?. If not, then there is nothing that you can do because at present there is no legislation that allows anyone to force someone to do anything about Japanese Knotweed on their land. If yes, then you need to initiate civil proceedings regarding its removal & the legal procedures will allow you to ask them for disclosure of all documents.

Have you tried to speak to your buildings insurance company ? speak to them and put them on notice saying there is this problem and that if it became a problem on your land they would be responsible for it and in their interest to speak to Network Rail ?. If it has encroached onto your land and you have buildings insurance, your insurers should be dealing with this ?.