 It was a little after 6 this morning that a tractor-trailer caught fire on the outer loop of I-495 in Maryland, causing major traffic delays. Montgomery County Fire and Rescue spokesman Pete Perringer said it appeared a vehicle collided with a tractor-trailer and then left the scene. He says that big rig caught on fire because of a ruptured saddle tank. About 15 to 20 barrels of crushed tungsten metal spilled onto the roadway of I-495.