Why not use a more modern mode of transportation such as monorail?
A major reason that monorail would be inappropriate is that the infrastructure for a rail transit service is already in place and such a service could be started relatively quickly at low cost, whereas the infrastructure for a monorail would have to be built from scratch and at a very high cost. Moreover, although monorail is a great concept, at least in theory, it suffers from some serious practical problems, including cumbersome switching arrangements and difficulty in running at ground level and in tunnels. Monorail is costly not only because it can only run on elevated beams but also because almost everything has to be custom produced and because there are very few suppliers and thus little competition. These are some of the reasons that almost all high capacity transit systems in the world, including those under construction, use steel wheels on steel rails. By the way, monorail really isn’t anything new — the first monorail was built more than a century ago.
