Many modern cities would crumble under our feet if they could not provide fast and reliable way of transport for millions of their citizens traveling to work or simply wanting to explore the city whose streets were not designed to support ever increasing number of automobiles. Solution for this is called Subway railway system.
Rapid transit in urban environment appeared in second part of 19th century, when London officials saw the need to enable faster travel for the citizens of city that was unable to cope with sudden population boom. From that moment on, rapid transit systems evolved and spreader across entire world, enabling millions to use trains on daily basis.