impassable

Incapable of being passed over, crossed, or negotiated.

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.

[...] 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.

Incapable of being overcome or surmounted.

Not usable as legal tender.