I just went in and created a server for Mission 4 (the "Train Mission").
It's still too laggy.
So to fix it I'm going to "nuke it from orbit".
Basically, the scrolling terrain is a treadmill system running in the background. (I wish I could convert all those entities to client-side only, in which case you'd have absolutely beautiful 3D scrolling terrain with no network traffic - but that's something to do if the Kickstarter campaign goes totally incredible.)
Basically, I'm going to rip it all out and just lay in a slab of bsp with a scrolling texture. That way the terrain will generate no net traffic, whatsoever. (Okay, I might put in train lights that whip by now and then...)
This is too bad, because the ending of that mission is actually funny...