In the early 90′s (and perhaps at other times, but I wasn’t there then), TWA added what were called “beyond fifth freedom” flights (an airline from country A operating and selling a flight from country B to country C) to its transatlantic schedules: for instance the New York JFK to Brussels widebody aircraft (a B747, B767 or L1011, depending on the year and season) would arrive in Brussels around 7 or 8AM, fly on to Berlin, Frankfurt, Munich or somewhere else in Europe (having let most of the New York passengers off at Brussels, and picking up some Brussels passengers flying to the European destinations). It would then pick up passengers at the other European destination, come back to Brussels by about 10 or 11AM, pick up the Brussels passengers and head back out to New York.

Usually it was the transatlantic aircraft which continued from Brussels to the other European destinations, in the case of Moscow it was a separate aircraft which flew the SVO-BRU-SVO legs due to time restraints. The idea was to optimize the use of the transatlantic aircraft (it was worth more in the air than on the ground) and to put additional passengers on the transatlantic leg (as Brussels did not fill the widebodies every day of the year).

Other European TWA destinations has similar “add-ons”, which had a tendency of changing according to season and year.

I can’t remember how successful the Moscow route was, but I do recall an incident of a Russian caviar seller calling the reservations office in a panic one evening because the flight had been cancelled and his caviar needed to be put in cold storage!