底铁

例句：

1. 1. The railway tracks are laid on a bed of concrete.
   铁路轨道铺设在混凝土上。
2. 2. The engineer inspected the subway system to ensure it was safe for use.
   工程师检查了地铁系统以确保其安全使用。
3. 3. The construction crew worked tirelessly to build the new bridge over the river.
   施工队不懈努力地建设这座新桥跨越河流。