Relative to the train, and by extension the portal, the people are moving towards it at the same speed as the train relative to the ground, since the people are tied to the ground. I'm gonna work with the definition of momentum that equals it to the velocity of an object times its mass, and with the assumption that the portals conserve mass and momentum of the objects during teleportation, or with negligible losses. Having found that the momentum stays constant, and given the mass before and after teleportation is constant, the velocities relative to the portal are gonna be constant too. (p1=p2 <=> mv1=mv2 <=> v1=v2). And since the velocity of the people relative to the portal is the velocity is the train relative to the ground, and the velocity of the train relative to the ground is far bigger than the velocity of the people relative to the ground, the answer is gonna be B, where the people shoot out of the portal with great speed.

If the people actually go into the portal and not under it that is.