T-Birds and Choppers are a little harder to come by for the average runner, but if you ask the right people in the Nexus, you'll come up trumps.  Its probably one of the better ways to get into Pueblo and Ute, better than overlanders due to the limited amount of road options you have in the less developed regions.  The better T-bird'ers generally don't have too much trouble with the drone set ups of Pueblo, although the occasional one gets nabbed every now and then.
The Road Less Traveled

This is the section where the real information is.  The previous sections, everyone knew about already, but these border crossings are some of the more unique ones, and lend themselves to Denver's history.  You see, chummers, Denver wasn't always a city with 5 different countries bordering on it.  It used to be in the middle of the USA, and isn't built for borders at all.  Above ground, there might have been areas bulldozed and people relocated to make room for checkpoints and fences, but below ground, very little can be done.

Sewer Surfing

The Denver Sewer system is, by and large, unmodified and ignored borders, especially those in the inner city, like the Aztlan sector.  It would cost millions upon millions to rip out large tracts of sewer pipe, not to mention all the facilities they'd have to rebuild.  So, by and large, the sewer systems are still in place.

The problem is, that you can just lock them off.  Sewer is often chunky, and putting in gates to mark borders can only be so effective, ideally, you need people to go in there to clear the gates as floating parcels of fun get caught up and eventually, if left unattended, block off the piping altogether.  Sensors find it hard to operate in the environment, with not only a moist atmosphere but often dangerous and damaging chemicals present in the system.  Not to mention, all manner of nasties that call the place home in 2062. 

However, the sewers don't run everywhere.  There's a myriad running into Aztlan's Sector, although I'm informed they have sensors on the manhole covers, I'm yet to have it confirmed and I have doubts on the effectiveness of such a system.  Crossing the borders into the Northern Ute Sector, Both Sioux nation and Sioux sector, as well as the outskirts of the UCAS sector (that lay outside the FTZ) are all possible.  However, the regions immediately outside of the FTZ are generally (at least in the aforementioned lands) residential rather than industrial, so be aware of that before you come up out of a manhole cover stinking like god-knows-what in the middle of Ares's latest middle-to-high density housing development for visa holders.

Dangers down in the sewers vary from pipe to pipe, but Devil rats are abundant in the pipes where the current is slow and the bigger places with walkways and ledges.  There's the occasional swimming creepy crawly thing in there, as well.  You could catch a myriad of diseases and infections from the water itself, so make sure you get yourself checked out after any sewer surfing you do.  Sluice gates and sudden currents, along with drops, pumps and (the worst thing to get sucked into) water treatment facilities are spotted about the place, mind you don't end up trapped in one of those.  Some of the industrial areas pump water at boiling temperature into the sewer system, I don't have to tell you to avoid those.

Basically, read up.  Your friendly decker can get you a town plan which will hopefully have a reasonably updated sewer system, but that wont cover all the man-made hazards.  It also wont cover the older tunnels as well.  As I mentioned before, Denver's been around a while.  It has at least one other system that I know of, which isn't really in operation, although some of the older houses are still connected to it and it joins the newer systems in places.  Some of these tunnels are over 150 years old, and are made of BRICK.  Some are collapsed, others look like they're going to collapse, and some are absolutely riddled with nasties, but the most attractive thing about these tunnels is that while you still have some chance of getting detected in the main sewers (especially the bigger lines), these ones are pretty much ignored by everyone
The Mole People

While we're on the subject of the sewers, let me just inform you of one of the more notably horrors down there.  Its no surprise that people who try to cross borders underground aren't heard of again, but that's probably due to any number of reasons, the least of which not being willing to advertise you're an illegal immigrant.  However, in the Sioux Sector, there's a number of people who have reported (to me, at least, this SOF card makes people chatty) about a gang of sewer dwellers.
Page 3