Longest can mean two things (time and length) but in the coaster stat world it means length of track. The longest coaster in the world is Steel Dragon 2000 at 8133 ft, longest in the US is Fury 325 at 6602 ft.
The Incridicoaster at DCA is actually #7, even with how big that building is it’s not going to touch those coasters; Exhibition Everest is currently the longest coaster at WDW at 4424 ft, if this coaster is longer then Everest, that’s going to be one heck of an indoor coaster and there will have to be a portion of the ride (not just the station) in the old UoE building, or there will be no room for theming in the gravity building because the track is going to twist too much on itself.