Actually, since you set the polarity FIRST before entering the "loop" there is no possibility of a short.
Likewise, since you then set the polarity BEFORE leaving the loop there is no possibility of a short.
Of course this depends on the train fitting entirely within the loop as stated ad nauseum.
I'd say that Neil's statement is most accurate, but staggering a few inches is not much length lost.
It would be helpful Sarah to give us your idea of a maximum train length as compared to your shortest "loop" (minimum distance between insulators)
The better and more constrained the questions, the better the answers.
Greg