Update 1: A Metro source tells me "The wheels separated from a train between L'Enfant and Smithsonian on track 2."
Update 2: Hearing part of the brakes, a "friction ring" fell off.
Update 3: via @r0d3nt who is apparently on the crippled train "#wmata is accessing panels to open doors. Some doors are jammed and wont open, intercoms non-functional."
Update 4: From a Metro source: "Word just came in that a collector shoe separated from the train, but I am also hearing friction ring ... We can't tell ourselves the truth"
Via @WilliamsMichael @unsuckdcmetro the evacuation begins! pic.twitter.com/11Q6g3UO
Update 6: via @IAFF36 "*U/D: Orange line train lost an axle, #DCFD units assisting approx 350 to Smithsonian station, expect major delays on Orange line #wmata"
Update 7: via @Aaronthepriest "Off the metro and in a cab. No shuttles there and @wmata made us pay exit fare!!!! @unsuckdcmetro @wmata is going to owe me my cab fare"
Update 8: More than 2 hours later and people are still on the train!
Update 9: Back to reports that it was a friction ring (part of the brake) that fell off. Wonder if we'll ever know what really happened.
Update 10: Another view of the evacuation via @Somethngfab
Update 11: Metro says they're doing track inspections, which is the final step before restoring third rail power.
Update 12: Trains are moving again albeit with single tracking
Update 13: Metro will hold a press conference at 3.
Update 14: From a very reliable Metro source "A brake disk (200lbs) broke free. Hit train behind it. The disk more than likely broke free and damaged that train but then looks like it possibly did damage to the train behind. There are either 8 or 12 bolts that hold the disk on. The bolts must have sheared off. Possibly inferior bolts from low bid contractor."