A couple weekends ago, my wife and I packed up our dog and headed to Philadelphia. I ran a race, but we also spent some time checking out the tunnel system under the city, known as the Philadelphia Concourse. I was reminded of that yesterday evening on my way home, as I wished for a tunnel that would take me under certain still-icy stretches of Baltimore's street grid.
I won't get that wish anytime soon, but I can at least write about what I learned in the concourse.
View Philadelphia concourse - key stops in a larger map
While there is indeed a thriving mall integrated into the system, along with a very nice train station, I wouldn't advise using the general tunnel system as a leisurely strolling area, nor even a place for jogging. You might save yourself a runny nose or two during cold months, but the lack of sunlight isn't worth it: With a few exceptions on the map above, the tunnels are grim and depressing.
That said, the lack of life underground might actually be a good thing for the city. A common idea in planning circles these days is that pushing people out of enclosed walkways and onto the street level keeps foot traffic heavy for local shops, which in turn keeps neighborhoods strong.
In addition to the critiques, I've heard some good things about these systems, especially in northern climates, where it truly can be too cold to walk. (Have a favorite place to exercise indoors? Leave a note.)
In any case, the giant, grade-separated pedestrian system in Philadelphia, encompassing both strengths and flaws, is about to grow again, albeit not underground: The Pennsylvania Convention Center, which is hooked into the system via the old Reading Terminal, is putting the finishing touches on a new extension along what was previously two blocks of Arch Street. The wing opens March 4.