Answer :
As v talk about India ...every city in India is over populated .During rush hour usually city face a huge traffic, where many commuters c't reach there work in time . so,under central government urban development scheme every metropolitan city has to develop a plan to improve infrastructure with the joint partnership with central gov in such way metro train is one such project.
the metro train services are introduced in big cities because there are many people there who can not buy a car or are aged hence metro helps them to reach there destination whithout costing much money. also one reason ins space without occupying space metro can not be made hence it is made in big cisties not small ones