Definition of "impassable"

At the same time, the encroachment of vegetation proceeds apace, and broom and brambles have already made portions of the line impassable, even on foot.

1949 November and December, “Notes and News: Festiniog and Welsh Highland Railways”, in Railway Magazine, page 408

[...] at Cowley Bridge Junction, east of Exeter, both lines were impassable from about 1.30 p.m.—the first flooding of the junction for 36 years—and by the evening the water had risen two-thirds of the way up the platforms at St. Davids as Exeter faced its worst flooding for 60 years.

1960 December, “Talking of Trains: The railway and the Devon floods”, in Trains Illustrated, page 709