Las Vegas is one of the few cities where this can be done. It’s not on a river or the coast so the water table isn’t an issue, and there’s few if any conflicting tunnels, shafts, sewers, or conduits at the depth the subway will run. They’re not boring through bedrock either.
Wasn’t there awestern movie a while back where they tunneled under the town to collect gold dropping out of the gambling houses? Worked great till it rained too much.
How stable is the tunnel if not boring through bedrock? Frankly, I don’t know, but wouldn’t that be a consideration?
Won’t be long, they’ll be finding bodies down there also.