Need Help with a Strange Portal Issue

[Razumen]

I made a moving polyobject with portals on its sides to act like a train, and it works perfectly in my test map, but when I moved it over to the main map, whenever I cross over the line into the train from the outside, the sectors inside the portals seem to just...disappear. There's no vertical collision inside where the train should be, but there's still collision for the walls as if I was actually inside the linked sector. I can however noclip and walk to the sector that the portals link to and go in there, where things render and act normally until I step outside the train. Then it reverts to the above behaviour.

 

Here's some images for clarity:

 

Outside the portal:

 

As soon as I step over the line:

 

Noclipped into the portals' sector:

 

Pretty baffled here, I'm guessing it's something to do with the size of my main map, which is about 30MB now. I've attached the test map that works, and can send the other map separately if someone wants to take a look at it, due to it's size and the size of it's separate resources it needs to run properly.

traintest.7z