It's not the actual tunnel boring that's the issue as modern TBM's have turned that into an efficient and usually speedy thing of beauty. You do have ventilation, fire safety and egress requirements which above ground does not, but ultimately it's mostly the damn stations, which still require the street to be cut-and-cover excavated and/or mined(in rock), which for obvious reasons is wildly expensive and time consuming. But few will doubt the obvious benefits of a fully underground railway. New and exciting solutions are on the horizon though including exploring fitting the stations within the diameter of the bore so all you need to do is mine surface access without needing to dig a battleship sized hole in the ground (or mine it out of rock). This is most notably being explored in one of the Sepulveda single bore heavy rail options.
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