On UA 1279 IAH to BOS on 12/16 (B739), with blustery weather in the Northeastern US the Flight Map indicated that our speed at one point hit 686 mph. I was hoping to break 700 mph, but I could feel the engines reduced when we were in the 680 range. After deplaning, I waited for the pilots to ask a question. I approached a person in pilot's uniform and he said that he did not fly the plane (I assume he was non-rev?), but that the day before he flew the route and had a tailwind of 168 mph. Rather than waiting for this flight's pilots (who seemed to be waiting for all passengers to get off), I moved on to catch my ride. I started thinking, what was the maximum speed observed by other FT'ers on a commercial (Non-Concorde/SST) flight?