Where are Tunnels Needed The BorderTransportationSystem is for the most part out in the open and most of the route is not hampered by mountains or city buildings.  But, there are some of each, and places like El Paso, Texas or Nogales, Arizona may be better served if the system is buried under the earth, i.e., a tunnel. Multistory tunnels through a mountain are difficult to build and maintain, and thus if needed it will be better to make multiple tunnels; see Separate Tunnels. Multistory tunnels through a town are simpler as the ground can be excavated to the proper depth, the tunnel built, and then the ground and buildings or streets replaced over the tunnel, much as was done to create the NYC subway system.. { see the Wikipedia article for restrictions on Photos, for Security, Flooding, Litter, Rodents, and Platforms} Index of ... NYC Subway System {Wikipedia Article} Separate Tunnels