A subway, also known as an underpass, is a grade-separated pedestrian crossing running underneath a road or railway in order to entirely separate pedestrians and cyclists from motor or train traffic. The 'Public Subway' sign at Monument station 's entrance refers to the pedestrian subways. Tunnel and underpass projects beneath operating highways and railroads are among the most sensitive and complicated underground projects in terms of design, construction, monitoring, and, most importantly, safety. They can potentially call upon many different tunneling techniques depending on the. Several kilometres of subterranean passages and air raid shelters stretch out under the city. Underground tunnels stretch out for kilometres under Manchester - with many residents unaware of the historical passages lying beneath their feet. There are three main methods for constructing underpasses: Precast concrete units. Being responsible for this national heritage is both a privilege and a challenge. These include the Forth Rail Bridge, which.