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"Rail gun" could illustrate the way carriages are added to and remove from the loop.

In the technical literature, the term "rail gun/railgun" is strongly associated with electromagnetic propulsion methods:

http://en.wikipedia.org/wiki/Railgun

Ah, I was thinking of a Gatling gun (http://en.wikipedia.org/wiki/Rotary_cannon), not a rail gun. The concept of rotary tubes seemed to mesh with OP's vision.

Or a continuous maglev force (as the OP wrote) for the entire track, augmented with air pressure to store acceleration energy in the system for the other trains.

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