If you go to DL.com and do a timetable search for FWA to DCA, quite a few unusual routing combinations come up - many with 3 flight segments. You'll hafto decide, based on the schedule and the amount of miles the new flights will earn you, if it's really worth your time.

I recently changed via SDC from PIT/MSP/DEN to PIT/DTW/SLC/DEN for some extra miles. My DTW/SLC flight was late due to an unruly, drunk pax who started walking around while taxiing. We went back to the gate to eject him - arrived late to SLC - missed my flight to DEN - got on the next one 90 min later - arrived in DEN at peak rush hour and ended up wasting a whole day flying!! - - But I got the miles - AND upgrades!!!