To make it clear from the start: I agree 100% with B - there has to be movement, because without it, people wouldn't come out of the portal at all. And if there is a movement, then the only reasonable speed would be that of the train.
But: Your question about the energy is still interesting. It must come from somewhere. And I think, the only source, from which it can come, is the train. That is, the train would lose energy and therefore slow down.
Bei mir gibt's die im Rewe, unter anderem auch am Stück. Ist bisher auch die einzige, die ich probiert habe, die eine zumindest etwas festere Konsistenz hat.