I think it has to be A. You figure that if it were B, the people on the track would suddenly be traveling at a high velocity, but the train’s velocity wouldn’t be impacted at all, since there was no impact between the train and the people. Wouldn’t this mean that the portal had created energy, which is impossible?

Portals already are infinite energy machines. Just out a portal up higher than the other one, and you get infinite energy generation via gravity.