I was surprised to learn that KLM flies non-stop three times a week AMS <-> IAH on a B737-700! These are flights KL664 IAH to AMS and KL663 AMS to IAH.
Didnt think that any B737 had a range of 5000 miles but I guess I was wrong. do they need special ETOPS certification, extra fuel tanks etc? The aircraft is all biz class, so there are not as many pax on board.