Why railway proximity comes up at all
Railway corridors are long, continuous and only intermittently managed, which makes them a recurring context for invasive species. Conveyancers have written about the combination for long enough that it exists as a distinct piece of Report on Title wording rather than as a footnote to the general knotweed paragraph.
Proximity to a line raises other questions too — noise and vibration among them — but the knotweed point is separate and turns on the land rather than the trains.
What can be established before exchange
Whether anything has in fact been identified, which is a question for a surveyor instructed to look at it. Where the boundary with railway land actually runs, which is a title question and not always where the fence is.
Whether the property has any history of treatment of its own. None of this depends on the railway operator telling you anything.
If knotweed is found near the railway boundary
The position becomes the neighbouring-land question rather than a railway question: how close it is, who owns the affected land, whether anyone is managing it, and what your lender's distance threshold is.
Where the affected land belongs to a railway undertaker, the practical difficulty is the same as with any other neighbour — you cannot compel treatment of land you do not own — but the scale of the landowner can make correspondence slower rather than faster. Plan for that in the transaction timetable.