track circuit permissive block

例句：

1. 1.The track circuit was designed to detect the presence of a specific signal.
   这个跟踪电路被设计用来检测特定信号的存在。
2. 2.The permissive block was used to allow for some flexibility in the design.
   这个许可块被用来在设计中提供一些灵活性。
3. 3.The engineers carefully monitored the track circuit to ensure its integrity.
   工程师们仔细监控着跟踪电路，以确保其完整性。