(~s)
1.
A ~ is an underground railway. (mainly AM; in BRIT, use underground , tube )
...the Bay Area Rapid Transit ~ system...
I don’t ride the ~ late at night.
N-COUNT: oft N n, also by N
2.
A ~ is a passage underneath a busy road or a railway track for people to walk through. (BRIT; in AM, use underpass )
N-COUNT