When I first started with United in 1995, it was commonplace to work a scheduled DC-10 flight between ORD-DTW and ORD-OMA. In fact, DEN-OMA service was also on DC-10's, if my memory serves me correctly. In the days of Shuttle by United, with SFO-LAX departures every 20 minutes, a bad weather day in SFO would cause multiple (full) Shuttle flights to cancel. Out would come the 747-400's from the maintenance base to operate 4-5 scrapped Shuttle flights between SFO and LAX...that was always fun.
There was a brief time period back in the late 1990's when United would position a 747-400, fresh out of SFO maintenance, to JFK so it could operate the JFK-NRT flight. The Premium Trans-con 767 flight would depart from SFO at 10:30 pm, filled to the gills. The 747-400 positioning flight, which operated 30 minutes later, would have no more than 30 passengers booked on it. We would work the 11pm all-nighter to JFK, have about 13 hours in NY, then work a 747-400 that had just come off the NRT-JFK trip, as a positioning flight back to SFO (for its scheduled maintenance check-up). The return to SFO was much of the same: The 6:30 pm dinner flight (a 767) would depart for SFO, filled to the gills, and our 7 pm dinner flight (the 747) would have about 30 passengers booked on it. The best trip I ever worked in my UA career, was one of those JFK-SFO 747 positioning flights. We had a minor mechanical problem, in which all 30 passengers were re-accommodated on the 6:30 pm departure to SFO. Here we were, a crew of 12, with galleys fully catered for 30 First Class and Business Class customers, and no passengers...ferrying the aircraft back to SFO. Debauchery and hedonism ensued that night...the entire way home to SFO. A good time was had by ALL.