Next time you drive on I-195 east of Providence, look to the north as you cross the river from Providence to Shelbyv-- er, East Providence, and you'll see a train bridge in the "up" position. How do I know it will be up? Here's the view looking back toward Providence from the bridge:

Unless a spectral locomotive comes through, there ain't no trains runnin' on this track.

After a short walk from the bridge through the woods, you'll get to this blocked-off tunnel. Along the top of this picture is Gano Street on the East Side of Providence, by the Dunkin' Donuts.

A closer look at the bottom-left. That's where you can go through. (I wonder if this tunnel connects with that "East Side Transit Tunnel" that only buses are allowed in. Ryan?) Kim and I did not venture that far. We took a left here and climbed the hill back up to Gano and walked all the way back to where we parked the car, at the soccer fields by the bridge.

It's a quick, fun, (legal?) urban adventure, complete with optional danger at both ends: The tunnel on the woods side, and the opportunity to walk out onto the bridge on the water side. (Look very closely in the top pic and you can see a guy with a Wu-Tang tattoo sitting out on the bridge. You won't see the tattoo but you'll see the guy.) The bridge walk is pretty safe, but you could go one step further and climb up the "up" part--I have read that it has been done. There's also a graffiti fun-zone out at the upthing, which you can see online if you do the proper web searching at ask jeeves or lycos or what have you.