Motion is relative though. Both the portal and the people are presumably on earth (or any other planet) and will thus be moving around in space at a fast pace. So the people have momentum, just not relative to the earth (or I guess more specifically not relative to the specific patch of earth they're on).

So there's 2 options:

• Portals do not respect the laws of relative motion -> the people will get yeeted in some direction because the earth they're on goes very fast through space and the portals are not pointing in the same direction
• Portals do respect the laws of relative motion, in which case the people will get yeeted with the speed of the train