What do you mean by being high enough density to support them? Light rail is rarely as economical of a solution as many suggest simply because of the routes it has to take in some cities and going underground just raises the cost exponentially. The cost per mile to develop and maintain rarely comes even close to efficient of above ground solutions.  Portland is looking to spend two billion on a line simply because it does go underground. There is no possible pay back.<p>So don&#x27;t discount bus service and other alternatives. This romance with rail, either above or below ground, sucks up way too much funding for what it delivers.