KLMBlue: Lufthansa does very well on this route. Of the three times I have taken this flight (LH422/423) they were full. Lufthansa has a daily 747-200, and during the summer has an additional flight (LH420/421) using an A340-300, up from an A300-600 two years ago.
However, United is looking to start flights next year using the 777, so we may not get the second Lufthansa flight, as they opt to code-share on the UA flight.
I do know that US Airways tried a BOS-FRA route a few years ago, but it was dropped after the Summer season. Apparantly it didn't do as well as planned, but I think that was due to the lack of advertising. I didn't even know of this route, until after it was cancelled. However I did see the US 767s landing a few times.
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