the "loop" is likely at the end of each line so the pods can turn around.

did you read the paper?

Not every word, I have a day job. If you have something specific to contribute please share.

The proposal is to use turntables. Since emphasis is made in the paper on maximizing turning radii, it could be that the train is not intended to be able to turn sharply enough to negotiate a feasible "loop".

loop /lo͞op/ Noun A shape produced by a curve that bends around and crosses itself.