These are rumoured to exist for the benefit of space travel during a game of Mornington Crescent. (See also: [Intergalactic Crescent]?
) However very little is known about them. There are vague statements in the literature suggesting that they stop at Saturn, Berlin and Marble Arch, and that air routes must be declared open before space routes can be, although air routes don't have to be open at the time of using space routes.
More concrete information would be welcome.
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