underground trackless transportation

例句：

1. 1. The train runs on the underground track.
   火车在地下轨道上运行。
2. 2. The subway is one of the most efficient ways to travel in the city.
   地铁是城市中效率最高的出行方式之一。
3. 3. The underground tunnels are used for emergency services during emergencies.
   在紧急情况下，地下隧道被用于提供紧急服务。