a single-track mind的用法和样例：
- A single track system in 1910, it was completely electrified and double-tracked by 1983.
- A single track system in 1910,it was completely electrified and double-tracked by 1983.
- Seen from front or rear, the forelegs, as well as the hind legs below the hock joint, move perpendicularly to the ground, with some tendency towards a single track as speed increases.
- The letter was written on a single sheet of paper.
- He got over the stream with a single leap.