Continental is not quite the biggest US carrier in Europe. Delta has the largest
transatlantic operations, from its JFK, ATL, and CVG gateways.
American is probably second (not in cities served, but through the number of
passengers it flies into London Heathrow).
Continental is probably third and closing in on AA. It serves more cities in the
Europe than any other US airline other than Delta.
Rounding out the top 10 are United, US Airways, Northwest, and TWA.