One of the cooler things you can do on a bike is ride through an abandoned and converted railroad tunnel, and in DC we're pretty lucky when it comes to that. We have a few good ones in the immediate area, like the Dalecarlia and the Air Rights tunnels on the Capital Crescent Trail and the Wilkes Street Tunnel in Alexandria and some of the country's longest ones within driving distance. In fact two of the 10 longest ones in the world (more on that in a minute) are not far away in Pennsylvania. They're old turnpike tunnels that started out as railroad tunnels even though trains never went through them. In addition the longest bike/ped tunnel in Virginia is in the DC area, but it's not a railroad tunnel.
Most of the longer tunnels for walking and biking are old railroad tunnels, but there are lots of shorter tunnels - we might think of them more as underpasses - that are used to help create a grade separated path for cyclists and pedestrians to get underneath roads or railroads. I've spent a significant amount of time making as complete a list of bike/ped tunnels (excluding those that can also be driven through - sorry Zoo tunnel) as I could, and I think its perhaps the most complete list anywhere. It is not, however, complete. There are likely hundreds of short underpasses that I didn't include, simply because the task of finding them is overwhelming. But I got all the tunnels I could find. Rails to Trails has a report on many of these tunnels, which while incomplete and hampered by errors, was the best existing list and was immensely useful.
There are some issues with my list. First is in deciding what is and isn't a tunnel you can bike through. The Slavošovský tunel in Slovakia is the 3rd longest in the wold. You can definitely ride a bike through it - because I've seen video of people doing that, but on one end you have to carry your bike out of a 20-30 feet deep hole to keep going. Not exactly a commuter route. Another problem is with getting accurate measurements of the lengths. In some cases I could find reliable measurements (from an engineering survey for example) but in others it's just a reported number (sometimes one of many and usually at a nice round number like 200 feet). So if you're hoping for final rulings on the relative position of tunnels of similar length, you're probably out of luck. Third is that I likely missed some tunnels, especially overseas. Known unknowns.
Having said all that, below is a list of the 10 longest bike/ped tunnels in the United States (for the international list, go here).
- Snoqualmie MR#50 (11894 ft) King and Kittitas Counties, WA John Wayne Pioneer Trail
- St. Paul Pass MR#20 (8976 ft) Montana and Idaho Route of the Hiawatha
- Sideling Hill (6782 ft) Breezewood, PA Pike to Bike Trail
- Norwalk (3810 ft) Sparta, WI Elroy Sparta State Trail
- Rays Hill (3532 ft) Bedford & Fulton Counties, PA Pike to Bike Trail
- Kennerdale (3350 ft) Kennerdale, PA Allegheny River Trail
- Big Savage ( 3295 ft) Somerset County, PA Great Allegheny Passage
- Paw Paw (3118 ft) Old Town, MD C&O Canal
- Rockland (2868 ft) Rockland, PA Allegheny River Trail
- Burro Schmidt (2640 ft) Kern County, CA Burro Schmidt Tunnel Trail
Of these, 9 started as railroad tunnels and one as a canal tunnel. Two of the railroad tunnels were converted to highway tunnels making them rails-to-highway-to-trails. The Burro Schmidt tunnel is a pedestrian-only tunnel. So 6 out of 10 of these are in the mid-Atlantic area.
But what about the longest tunnels in the closest 5 states to DC (MD, VA, PA, WV and DE)? Well, Delaware doesn't have any bike/ped tunnels (yet) - in fact I don't think there's a single tunnel in Delaware at all. In addition to the 6 above, the next four are all in West Virginia, with three of them being on the same trail:
7. Central Station Tunnel (NBRT #6) (2297 ft) Central Station, WV North Bend Rail Trail
8. Eaton Tunnel (NBRT #21) (1840 ft) Eaton, WV North Bend Rail Trail
9. Knobley Tunnel (1448 ft) Carpendale, WV Carpendale Trail
10. Silver Run Tunnel (NBRT #19) (1376 ft) Ritchie County, WV North Bend Rail Trail
Finally, here's the list of the longest tunnel by state. Note that, like Delaware, not every state has a bike/ped tunnel. The number refers to it's rank among all bike/ped tunnels. Also, even though the St. Paul Pass Tunnel crosses from Montana to Idaho, I counted it as neither's longest.
31 Moss Creek Tunnel MR#22 (1516 ft) Roland, ID Route of the Hiawatha
32 Cartwrigth Tunnel (1458 ft) McKenzie County, ND FB&CT Trail
34 Chetoogeta mountain tunnel (1447 ft) Tunnel Hill, GA Chetoogeta mountain tunnel trail
40 CNTP Tunnel #9 (1290 ft) Alpine, KY Cathy Crockett Trail
45 Tunnel On Lakewood Drive (1200 ft) Swain County, NC Lakeshore Tunnel Trail
47 Little Tunnel (1193 ft) Cumberland Gap, TN Little Tunnel Trail
89 Clarity Tunnel (630 ft) Caprock Canyons, TX Caprock Canyon Trailway
92 Seaboard Airline Tunnel (600 ft) Columbia, SC
101 The Tunnel at Tunnel Hill (543 ft) Tunnel Hill, IL Tunnel Hill Drive
102 Pentagon Tunnel (536 ft) Arlington, VA
105 National Road Tunnel (522 ft) St. Clairsville, OH National Road Bikeway
116 Dominion Tunnel MR#19 (474 ft) Saltese, MT Route of the Olympian
124 Tunnel Through Ely's Peak (450 ft) St. Louis County, MN DWP Trail
126 Oswego Tunnel (429 ft) Oswego, NY Harbor Rail Trail
137 HRT Tunnel #3 (383 ft) Boulder City, NV Historic Railroad Trail
141 Tunnel Drive #3 (376 ft) Canon, CO Tunnel Drive Trail
142 Mosier Twin Tunnels (369 ft) Mosier, OR Columbia River Highway Trail
165 Portland River Trail Tunnel (277 ft) Portland, MI Portland River Trail
167 Tunnel A (270 ft) Fall Draw, SD George S. Mickelson Trail
174 Foundry Branch Tunnel (252 ft) Washington, DC Capital Crescent Trail
177 Rocheport Tunnel (243 ft) Rocheport, MO Katy Trail
179 Shepaug Tunnel (235 ft) Roxbury, CT Steep Rock Trail
225 Saddle Rock Tunnel (76 feet) Scotts Bluff National Monument, NE Saddle Rock Trail
228 Phipps Tunnel (70 feet) Holliston, MA Upper Charles Rail Trail
239 Lafayette Street Pedestrian Tunnel (55 feet) Tallahassee, FL
Most of these are old railroad tunnels, but a few are not. The tunnel on Lakewood Drive in North Carolina is an old road tunnel as is the Mosier Twin Tunnels in Oregon. The Pentagon tunnel was, of course, built specifically for pedestrian traffic as were the Portland River Trail Tunnel in Michigan and the Lafayette Street Pedestrian Tunnel in Florida. The Foundry Branch Tunnel in DC was originally a stream/sewer culvert, then a road tunnel. Finally, the Saddle Rock Tunnel, which is on a hiking trail, was a test tunnel for a road.
Also of note, the Shepaug and Chetoogeta mountain tunnel are pedestrian only, with the latter being part of a museum. You have to pay to walk through it and then all you can do is hike to the end and then hike back.
By the way, the Pentagon Tunnel is the 2nd longest specially built bike/pedestrian tunnel in the world, behind only the 1330 foot Mount Baker Ridge Tunnel in Seattle, Washington.
Finally, here's the information for the tunnels in the immediate area. Again the first number is their rank in the world.
68 Air Rights Tunnel (800 feet) Bethesda, MD Georgetown Branch Trail
102 Pentagon Tunnel (536 feet) Arlington, VA
143 Wilkes Street Tunnel (360 feet) Alexandria, VA
149 Dalecarlia Tunnel (340 feet) Brookmont, MD Capital Crescent Trail
155 Holmes run Trail Tunnel (300 feet) Alexandria, VA Holmes Run Trail
172 Foundry Branch Tunnel (252 ft) Washington, DC Capital Crescent Trail
173 Telegraph Road Tunnel (250 feet) Alexandria, VA
186 193 Tunnel (201 feet) Glenn Dale, MD WB&A Trail
193 Duke Street Tunnel (163 feet) Alexandria, VA
205 Crystal City Connector Tunnel (117 feet) Arlington, VA Mount Vernon Trail
208 Race Track Road Tunnel (108 feet) Bowie, MD WB&A Trail
229 Old Waugh Chappel Road Tunnel (65 feet) Odenton, MD WB&A Trail
232 Maryland Avenue Tunnel (64 feet) Rockville, MD New Mark Path
240 Center Leg Freeway Pedestrian Tunnel (49ft) Washington, DC
(Update: Since originally publishing this, readers have pointed out other tunnels - including two in DC that I missed - and this has been updated accordingly.)