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How can it not be b? Every situation in the Portal games is already exactly like this, but with the portal fixed to a slab that moves with the rotation of the Earth, whereas in the drawing the portal moves as the sum of earth rotation + the movement of the train.
Because the rule that's literally in the game
"Speed thing goes in, speedy thing comes out"
Something isn't moving goes in, it won't move coming out. A hole having momentum won't transfer it to what passes through the hole.
Basic stuff
But, "speedy" relative to what? Relative to the walls of the room your are inside? What if you are in a falling elevator? Relative to the rotating surface of the earth? To the center of the solar system? "Relative to the portal" is the only answer to that question that makes sense.
Someone watched a YouTube video and think they know what they are on about..
Portals don't transfer energy, there is zero energy transferred to the people they are simply moving from one space to another
But if we use coordinates relative to the orange hole, the whole world, including the rails and people on them, is moving, and the people are moving towards the hole with a speed of the train.
That YouTube video was wrong, doesn't even make sense
It's a hole, no energy gets transferred