@ceo_of_monoeye_dating @roboneko @mushroom_soup @ai @Inginsub @cassidyclown @Kirino Hmm.

First, I don't think momentum is conserved by portals (it is conserved relative to the portals, but not globally). Momentum is a vector quantity, meaning it's dependent on direction (which can get confusing in the curved space time of something like a wormhole). Objects exit portals relative to the normal vector of the second portal. So momentum can be conserved depending on the arrangement, but it can also be "reflected" or rotate the momentum vector 180 degrees to itself before it entered the portal. However, since no momentum seems to be transferred to the portal or the surface it's on (think throwing a rubber ball against a brick wall, a little momentum is transferred to the wall to reflect it), it seems that momentum isn't conserved.

The thing that does seem to be conserved is "speed" (but not kinetic energy). The question now is "speed relative to what?" I think it's impossible to say for certain, there are a lot of arguments to be made either way, but I think the easiest answer is "your speed relative to the first portal you enter". So in this case, I believe the people would fly out of the second portal when hit by the portal on the train.

Another example to think about is a portal in a falling elevator. While it's in free-fall, how would someone exit the portal? I don't think they'd be shot out of the portal if they exited it, so portals likely conserve your speed relative to the portal you enter.

Edit: A stronger argument for this, I just realized, comes from the fact that there is no time delay when entering a portal. You enter and exit both at the same time, instantly, based off the mechanics in the game. As stated previously, no momentum seems to be transferred to the portals/portal surface when entering or exiting them. This also means that no forces are exerted on anything that passes through the portals. If the "plop" outcome was accurate, there would either have to be a time delay between entering and exiting the portal, or there would have to be forces acting on the people, compressing them somehow as they enter and exit the portals.

In this example you could also make a spaceship that runs off a free energy engine, hence kinetic energy isn't conserved either.

In this example, the spring mass is tossed to the front of the ship, hits the wall and bounces off in a nearly elastic collision, transferring a little of it's momentum to the ship and causing it to move forward. The spring mass then travels back to portal 1 with a relative speed greater than it's initial speed (since the ship is moving now slightly). It exits portal two and hits the front of the ship again, adding even more momentum (since the spring mass is moving faster). Then it bounces off again, goes back into portal 2, and repeats the process.

Unless portals and the things that goes through them consume energy from the gun, portals violate conservation of momentum and energy.