I don't recall creosote ever causing a problem when used for walls or dock pillars in a pond. "Old" railroad timbers have been rained on for years with the runoff entering the waters surrounding the tracks without issue before, so I don't think you will have a bit of trouble from them being on your road. IIRC, most litigation was initiated by the EPA. I have seen RR Tie piles in the Kansas City bottoms that were 30 feet tall and covering 40-50 acres in an attempt to try infusing epoxy into the ties to seal the creosote and reuse the old ties, but what I never saw or heard of, was any fish killed in the Kaw River only a hundred yards or so away, where the piles drained into. When the pile caught fire during a lightening storm sometime around 1990, it burned for months and there is no telling how much water was pumped on it till it finally burned out...still, no reports of dead fish in the river.