single track; single line
Refers to a railway line with a single set of tracks, where trains in both directions share the same track.
This line is single-track, so passing each other takes time.
solid wire; solid-core wire; single-strand wire
Used in electrical contexts to describe a wire consisting of a single solid conductor, as opposed to stranded wire.
Solid wire is harder to bend than stranded wire, but connections are more secure.
single-track operation
single-wire line
Compound of 単 (single) and 線 (line/wire). The railway sense is the most common.