ABSTRACT
In developing countries metros are generally understood to refer to underground rail systems. 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. In London the metro rail lines are below ground in the city area and on the surface in the outskirts. BART (Bay Area Rapid Transit) in San Francisco revived the idea of using elevated structures for the metro over road medians for a considerable length (37 km elevated, 36 km in tunnel and 40 km surface, then). The cost of construction vis-a-vis aesthetics and space shortage on the chosen route now dictates choice of form of construction. The first phase of Singapore metro comprises a short surface route, a 41.4 km long elevated and a 20-km underground route. In Indian conditions the initial cost of construction would be in the ratio of 1:3:6 to 1:4:10 for the three forms (surface, elevated an underground).
