The original Mobile Lounges (see picture at
http://www.metwashairports.com/dulle.../mobile_lounge) transferred passengers from the Main Terminal directly to the planes. There were no gates and jetways at Dulles when it opened. The right end of the picture shows a maneuverable/extendable ramp that extended to the door of the plane. Those ramps were cut off as the Mobile Lounges were converted to Terminal-to-Concourse transportation - and the newer Plane Mate (
http://www.metwashairports.com/dulle...ery/plane_mate) was put into service.
The Mobile Lounge couldn't service the taller jumbo jets - the Plane Mate could service anything. They used Plane Mates to service the Airbus A380 when it visited Dulles a couple of months ago.