 It is the longest bus route currently operating in London, covering a distance of 23.75 miles, albeit with limited stops.Route X26 has its origins in Green Line Coach route 725, which started in the 1950s and ran from Gravesend and Dartford through Bromley, Croydon, Sutton and Kingston to Staines and Windsor.In the late 1970s route 726 was introduced as a variant, from Gravesend to Windsor via Heathrow Airport and Slough instead of Staines.