That's actually a really good question. Nobody knows yet, but I would guess, since the game appeals to a lot of users, both those who want to build everything from the ground up to those who just want a casual experience, that there will be some pre-built coasters.
Perhaps down the road, Brett and Zac can arrange a competition of some sort where the alpha testers can build coasters for a chance of it to appear as a pre-built coaster in the finished game.