hey, yeah, use the teleporter or warpzoneportal in your rollercoaster or bastardize them if you think it'd help ... i read that whole thread, fascinating.
I'd try to get involved only what little spare time i've got i'm absolutely immersed in trying to solve my problem of guaranteeing that mover faces can never disappear while playing my map ... Lots of movers in a small area, and after about three games in a row, one or two faces start disappearing. Seems to be a bsp thing, yet it doesn't happen consistently, but it's consistently similar regardless of the machine running it. Very weird that i can take more than a game or so to show up. Wish there was some sort of Repaint function i could call on the problem movers, bah.
Anway, the rollercoaster thing really looks cool, very creative, and it looks like by now you've figured out all the physics of the tunnels, but some of the early posts made me think of all the time i spent getting my Anti-Gravity lift operational. You can get some really weird results. But at the top of the lift the player is gently guided around a bend simply by the sloped architecture, the reason it's not a problem i guess is because the motion is locked into a fairly slow velocity (originally to allow time for melee and firefights inside the antigravity chamber), and for your situation you need high-velocity travel.
But it seems like at one point i had several different velocity zones in there and tried all sorts of things, many of which caused players to "stick" to the sides of the chamber, as if the walls were very sticky. It annoyed me to no end, but tinkering with the settings long enough solved it, and the only reason i have any extra zones at all now is for a special effect and a little damage that occurs halfway up. Something else to watch out for ... i don't know what the LowGrav mutator that comes with the game does physics-wise, but i think whoever wrote it was careless with some absolute values somewhere or something because it cancels out reverse gravity in some instances.