Thanks for the kind words, everyone! It feels good to finally be able to show this off to the public . . . and be this close to releasing it at long last.

Now to answer a few questions that have come up here and elsewhere . . .
Does this support freight movement?Yes. Not only can Cars use this for commuting purposes, but New FLUPs can also support Freight Truck and Bus traffic--a significant upgrade over the old Subway Converter lots, which could only accept Cars.
What underground networks are supported?Presently, there's only the "RHW-2" and "MIS" versions (which would be used for any 2-lane/2-way and 1-lane/1-way underground network, respectively).
Girafe has made RHW-3 portal models, though I've yet to implement those, and there are no RHW-4 portals as of yet.
Eventually, the plan is to support underground networks that mirror RHW/NWM/Base Network counterparts, though "RHW-2" and "MIS" are probably all that'll be in NAM 39.
Can these things cross regular Subways?Technically, no. They're at the same height level. An actual crossing would require either another sub-level for the underground roads, or the Subway itself (and resultant underground height transitions). There may be a way to cheat it a little, but that hypothesis has not yet been tested.
Can URail be reimplemented using the same technology?Sadly, no. Both
eggman121 and I tested a Subway-based URail replacement extensively, and unfortunately, Rail pathing does not play nice with Subway-based override networks, to the point of such an override being effectively unusable. It's a real shame, since URail is a particularly gnarly mess of puzzle pieces in its current iteration, and we are, of course, trying to shuffle as many old-style puzzle pieces into obsolescence as possible. We'll continue to look for such a way to do that, but that's a huge unknown at this point.
Can Pedestrians use them?That hasn't been tested, and the existing underground networks do not have Pedestrian paths on them (carried over from the existing FLUPs puzzle pieces). If we are to give an underground Pedestrian network a go, it'll be its own thing, without other vehicular traffic (if you're spending that much money to build a tunnel, there's no sense in mixing Peds and Cars).
-Alex