[mseebach2]

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

[lutusp]

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

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

[mseebach2]

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.

[manmal]

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.