Regionally, I can't think of any infrastructure that's under-utilized rather than over-utilized and under-maintained. There are a couple proposed projects that

Regionally, I can't think of any infrastructure that's under-utilized rather than over-utilized and under-maintained. There are a couple proposed projects that would be stupid to build in the first place, let alone over-build; but I can't think of anything existing that isn't already at or over capacity and very obviously going to be heavily used for the next 50 years and probably more. In fact, our entire transportation network is badly screwed and about to become far worse because major elements of infrastructure we've been using for between 70 and 120 years years and have taken for granted has seen increasing use but has not been sufficiently maintained. If it had been built with cost-saving instead of robustness in mind, we'd have had to spend 2/3 as much three times over to rebuild it... or more likely, not have been able to muster the political will to rebuild it at all and instead seen our economy fail due to the lack of infrastructure necessary to support economic activity. The same goes for our water delivery infrastructure (it's taking 50 years to build a new tunnel so we can even take the existing ones offline for their first maintenance in a century), our schools and post offices, and probably a bunch more things that aren't coming to mind just now.