Möbius [1993, Germany, sci-fi, SATRip]

A train and all its passengers disappear from the Berlin underground system, apparently travelling as a ghost train beneath the city. A young mathematician develops a theory: The train is still inside the network, but due to the opening of a new line, this network has become so complex that the train went into the 4th dimension and is travelling on the backside of a Moebius-strip. Unfortunately, also the designer of the network vanished...