I'm somewhat sceptical about rumours that Railtrack, or someone acting at their direction, could have deliberately infected embankment-dwelling rabbits with the myxoma virus.
I'm pretty sure you can't just buy 500ml of an active viral agent at your local pharmacy, on e-Bay, or even from Porton Down research facility ...
The only practical way of introducing the virus to rabbits, is to expose them to already-infected rabbits from elsewhere (transmission is via biting insects, normally the rabbit flea).
So that would entail introducing even more rabbits to an area defined as having a rabbit problem, in the hope that maybe the existing population of rabbits doesn't already have some immunity from the virus.
That's not exactly a fool-proof business plan, and not one I'd have much confidence getting past my bank manager.
Sometimes 'common knowledge' is spread, virus-like, from an initial supposition or rumour, until the truth of the matter can become undiscernible from myth.
The number and severity of myxomatosis outbreaks varies over time; the myxomatosis virus is notorious for its ability to mutate from year to year and the background immunity in the wild rabbit population also varies.
So most likely, the localized naturally occurring outbreak(s) adjacent to the railway property was just misinterpreted as something more sinister, by those folks who weren't in possession of the full facts, or able (or willing) to see the bigger picture ... it's how most conspiracy theories start.