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There's a bias to keeping them on the JFK service, though that's not an overwhelming bias, and there are some routes which only get the Super J by accident, presumably there's nothing else left in the shed at the relevant time. Lagos, Kuwait, DFW, SFO and San Diego seem to do well on them,. But apart from that I don't think there is a formalised system of routes that you can rely on. The Jet Tracker on thebasource gives some clues