Despite the benefits of rapid transit systems on urban environments, the metro would have to be precisely planned and routed. Without this consideration, considerable dislocation of local traffic and displacement of people made homeless by the demolition or damage caused to their homes and places of work would occur.
A careful and systematic route and the ancillary demands it lays on the surrounding city should also be considered, such as access for construction vehicles and workers, for instance.
Additionally, the metro should be routed to areas that really require the relief or utility of the system in the first place. Profit for any individuals should be secondary to the social use of the system and its utility.
Costs can overrun projects and this can cause a major deficit to a city's finances. In addition, where there are no large and wealthy populations able to pay for trips on a metro, the government may have to subsidize the operation and the fares for the metro.
The actual cost of the metro may not be recovered from its operation for many years, leading to further deficits caused by having to pay for the whole construction process in the first place.
Attempts to recover costs have already been made by the government of Karnataka, which has for the last decade been collecting taxes on fuel specifically to fund the construction of the Bangalore metro.
Constructing a metro may damage the local environment as the earth is moved and concrete transported, for example. The spoil from the underground excavations must also be considered because it needs to be disposed of. The carbon footprint of the work is also important, and must be considered.
Needless to say, politicians will inevitably use the whole project to either line their pockets or to boost their popularity. As a result, the whole project can suffer badly due to infighting and corruption.