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.