I'm not sure about boomerangs as I have never really closely studied them, but I do know that on the Invertigoes there is a brake between the 2nd lift and the vertical loop. It's primary function is to keep the train from being stuck in the loops if the 2nd lift malfunctions.
As for a re-start procedure, I dont *exactly* know what they do, but I would venture to say that it is not very complicated. One day while I was @ SFA, someone told me that Two-Face was stuck like that but when I went over there a couple of hours later, everything was fine. So I dont think that too much needs to be done.
jeremy
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"A little nonsense now and then is relished by the wisest men" - Willy Wonka: Willy Wonka and the Chocolate Factory