Suppose the blue portal is instead aligned parallel to and facing the ground. Maybe a 18" off the ground, a little higher than a person is wide. Additionally, the person is standing upright on the track.
In the above scenario, with the ground rushing at the person, does it suddenly “stop,” with the person gently falling onto the ground? This is the same problem, I suppose, but from a different perspective.
Now, what if that blue portal is instead only 6" off the ground? Is the person embedded in the ground, or does the universe crash?
I think you would collide with the ground like you were falling face-down, and is there isn’t room you’ll simply remain half-in half-out. At that point, your front half is still relative to the ground, but the back half is moving with the train, that way your velocity is zero in relation to both portals.
Suppose the blue portal is sitting upright on the tracks facing directly into the orange portal (parallel to it) from some distance. We will of course neglect gravity and most physical laws.
Option B: The people shoot out of the blue portal, eventually reaching the orange portal again after a finite time (exactly halfway between the portals). At which point, their velocity relative to the train is double what it originally was, and they shoot out of the orange portal again twice as fast. Since the people are faster than the train, they will hit the train before it covers half of the remaining distance; and so this all happens again, with the people’s velocity now increasing to triple the initial value. And it happens again, and again, until relativistic effects take over and the velocity is no longer approximately additive. In other words, the people accelerate to an appreciable fraction of the speed of light, regardless of the starting velocity.
Option A: The people plop out of the portal and eventually get smashed between the train and the portal wall in a satisfying and physically plausible fashion.
Bonus Option C: In option B, it is unspecified if the resultant velocity of the people is equal to the velocity of the people relative to the train, or equal and opposite to the velocity of the train relative to the people. This difference becomes meaningful at the relativistic speeds we achieved, and I implicitly assumed the latter. In the former case, the people are eventually carrying ~100% of the energy of the system and therefore doubling it every time they pass through the portal, and time dilation be damned, for an instant they achieve infinite energy.
Now suppose the blue portal is just a centimeter behind the orange portal, opening the other direction, so anything that goes in one almost doesn’t even seem to have teleported. When the people pass through the orange portal, they appear on the other side; inside the train.
Option B: As the people pass into the portal, they instantly shoot backward, as if the train grabbed and threw them behind itself.
Option A: The people simply pass through the portal, as if it weren’t there at all.
Except in that scenario both portals are moving if they act like a moving hole. Imagine a hula hoop, except it’s 2 portals connected back to back. If I passed a hula hoop over you, you’d be going into the bottom at the same velocity that you are coming out the top, therefore momentum is preserved. You’re moving at the exact same velocity in reference to both of the portals
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