Yes, that is called running extra fast. And then falling, with the same momentum. Unless there are two grounds, with different relativities. Like with a treadmill: you run relatively to the treadmill, but you are stationary relatively to the ground under it, because you run at exactly the same speed as the treadmill moves in the other way (hopefully for you…).
A.
its the train that has velocity. The people who enter the portal will not be moving?
Its like that buster keaton clip where he stands still and the side of the house falls down around him(well… sort of)
The train has absolutely no velocity relatively to the orange portal. The people are moving relatively to the orange portal.
If the ground disappears from under your feet at 60 miles per hour, the moment you start falling are you falling at 60 miles per hour?
Yes, that is called running extra fast. And then falling, with the same momentum. Unless there are two grounds, with different relativities. Like with a treadmill: you run relatively to the treadmill, but you are stationary relatively to the ground under it, because you run at exactly the same speed as the treadmill moves in the other way (hopefully for you…).