is
True when nothing in the model will make this transporter move again by itself: it carries nobody, has no route under way, and is not itself waiting for anything.
One definition, in one place, because two diagnostics now rest on it. The deadlock detector uses it to tell an obstruction from a circular wait, and the guide path uses it to say whether space a holder is waiting for can ever drain. Those must never disagree: a run that reports "still draining" for space that a parked vehicle will occupy forever has told the modeller the one thing that is not true.
It is a judgement about a single instant and can be overtaken by events -- an entity may seize this transporter a moment later -- which is why both callers warn rather than raise.