HS4Air is a proposal for a 140-kilometre (87 mi) high-speed railway line in the United Kingdom, put forward in 2018 by a British engineering consultancy, Expedition Engineering. The proposed line would have connected the planned High Speed 2 line to the High Speed 1 line via a high-speed route running south of London, and would have formed a direct rail link between Heathrow and Gatwick airports. The proposal was rejected by the government in December 2018 and will not go ahead.