Metro rail systems need not necessarily be entirely underground. A major part of the metro rail in New York and Chicago has been and is being constructed on elevated structures. Metro or subway tunnels refer to a continuous underground passage provided for movement of commuter trains within cities and suburbs. The cost of construction also depends on choice of method of construction, viz., cut-and-cover or bored tunnelling. Tunnels can pass under built-up areas where curves are introduced but would have to be well clear of the foundations of buildings, that is, below the zone of influence of such foundations. For the bored tunnelling method, space between tunnels has to be cleared to accommodate station platforms with the concourse placed either on surface or underground. The construction of tunnel boxes using cut and cover method requires opening part of the road and keeping it unavailable for road traffic.