But U.S. citizens can also join by applying for the U.S. Customs and Border Protection (CBP) Global Entry program -- a "trusted traveler" scheme in which individuals sign up to be vetted in order to expedite the customs process on arrival into the country from abroad.

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Remember that the Eurostar station at St Pancras International does passport checks prior to departure, which can surprise travellers expecting the customs process to take place at arrival.

BBC: Navigating the London 2012 exodus

The Pentagon would also speed up the arrival of replacements by a month or more, a process that could continue into the new year.

NPR: Speculation Mounts on Troop Additions in Iraq