a train indicator的用法和样例：
- A car journey is as fast as a train journey.
- Don't jump off a train when it's on the move.
- Owing to sabotage, a train jumped the rails.
- The pop star was followed by a train of admirers.
- I'll have to ring off now, I have a train to catch.
- I have to ring off now. I have a train to catch.