This idea was put forward for discussion at the Duxford Creative Workshop in July 2009. The short description is ;" A vaccuum would be created in a tube system that permitted electrical propulsion with much less air resistance. Could be combined with a Maglev system for even higher efficiency." This accouint is a later extension of the basic idea.
Today many airprorts lie at considerable distance from the heart of the cities they serve and transit between the two is a significant [part of the whole journey time. Other methods have been used to cut down this time loss for the passenger. These include (a) non-stop faster tube service (London) (b) very high speed train (Beijing and Hong Kong) but this idea offfers a different solution.
The basis of the idea is that a suibstantially air tight tube be constructed going undergound (or in some places - on the ground) between the two locations. Air would be evacuated ahead of the train to create a poistive air pressure behind it and this would contribute bot to driving the train and to reducing the forward drag experienced by it. Depending on the bore of the tube, the sealing and pumping arrangements and the length of the section the air pressure differential may be adequate to accelerate the train to high speeds. In locations where this is not achieveable the concept could very well be combined with any high speed train system but the Maglev concept seems particularly well suited giving very high speeds with an extremely smooth ride.
The basic idea is not new and several forms of sealed tube working either under pressure or under vacuum have ben mooted or trioed previously. The challenges of making an efficient and operable design include : Designing the vacuum pumps to evacuate the tube ahead of the rail car efficiently, creating an adequate seal between the rail car and the stationary tube - including overcoming the rail car-rail car joints, designing the sealing mechanisms ahead of the rail car to sustain the pressure differential, partitioning long sections of track into a number of zones in which the low pressure effect would work and be passed from one section to the other.
The most practical thought for the passenger stations would be to include at least some form of self propulsion capability that allowed the train to move off from the station and accelerate a little through the first pressure seal so that this could then be de-pressurised and the main acceleration of the car achieved.