I am in Chicago currently and have noticed that a considerable part of their Metro system runs above ground elevated on metal girders (think the Loop Line all over Dublin).
As an efficient use of space, it is second to none, requires less disuption to traffic to build and provide visually appealing public infrastructure.
I can only imagine the response of Frank McDonald of the Irish Times to my suggestion. Probably give the man a heart attack. Of course, that could be seen as an ancilliary benefit!