That is assuming both portals have speed. If the other portal was on the back of the train it would be like a hula hoop but if the other one is stationary you have the people going in really fast and coming out of the stationary portal at that speed.
Why?
Where does the energy even come from?
A hole/portal doesn’t create or generate energy it just passes things through.
Just think of it as a hole across space because that is exactly what a portal is.
It’s already established in Portal that energy can be created by changing elevation with portals. If the other portal is on the back of the train, you’d just fall out because you’d be shooting backwards at a speed that cancels out the speed of the back portal. If that portal were just sitting stationary you’d have to fly out at the speed you went in. The puzzle comes from people trying to apply ordinary physics to something that deliberately breaks them.
No energy is every transferred as a result of a portal
You fly in the air if you drop in one because you are carrying momentum downwards that suddenly translates to upwards
You are sat in the floor, a portal flies towards you. You are sat at the floor at the end, you had no momentum going in and no momentum going out
