I agree, in the portal game they would be scraped/blended on the bottom of the support of the portal attached to the train. If they were in fact hovering in line with the portal I’m leaning towards, an object in motion will stay in motion. Given portal orange is moving and blue is stationary, objects entering the portal will exit at the same velocity and b would probably happen in game.  

This is going to be awesome! I really loved Sync and am looking forward to it again :)