Advantages of Underground Rail

Well, if I'm going to talk about the advantages of elevated rail, it feels only fair to discuss some other advantages: like the advantages of the other main grade-separation option, underground rail.Because if there weren't any, well, we'd all build the cheaper, elevated option, wouldn't we?Note that when I say "we," of course, I don't mean the Quad Cities. Lord no! It would be nice, but let's get some basic bus frequency and maybe a rail link to Chicago first.But "we" in the larger sense do keep building tunneled, or cut-and-cover, rail: so why? What is it bringing to (or rather, under) the table?1. Land UseProbably the most obvious advantage of underground rail, besides grade separation itself, is that it has limited demands on the surface: while it requires just as much space as anything else when you consider the space underground, we as a society tend to value the surface much more than the depths: we live there, after all.That means that a form of transportation whose visible…