By 1957, it was obvious that transatlantic travel was becoming dominated by air travel due to its speed and low cost relative to sea routes, with passenger numbers split 50:50 between sea and air transport. The increase in market share by air showed no signs of slowing down, especially once the Boeing 707 entered service in 1958. Conversely, and "Queen Elizabeth" were becoming increasingly expensive to operate, and both internally and externally were relics of the pre-war years and needed to be retired by the mid-1960s.