tunnel obstruction signal

例句：

1. 1.The tunnel was blocked by a large rock, causing the train to derail.
   隧道被一块大石头堵住了，导致火车出轨。
2. 2. The engineers used a tunnel obstruction signal to alert the crew of an impending danger.
   工程师们使用隧道堵塞信号来警告机组人员即将发生的危险。
3. 3. The tunnel obstruction signal was a crucial piece of equipment in ensuring the safety of the construction workers.
   隧道堵塞信号是确保施工工人安全的关键设备。