The other day I was daydreaming and I had an idea for a roller coaster pop into my head. Of course, I’d be surprised if I haven’t already been beaten to the punch on this.
As you can see from this footage, when the coaster’s train is moving up, the rider’s car is pitched backward at an angle; when the train is moving down, the rider’s car is pitched forward at an angle.
But what if, through some feat of engineering, the rider went through all the hills & swoops and whatnot, but the rider’s seat remained horizontal throughout the entire ride? IOW, if I were riding, when going up a hill my seat would move upward, but the angle at which I’m sitting would remain precisely 90 degrees; ditto for going down. It would require some finagling but I don’t think it would be beyond the limits of modern technology.
Anyway, has such a thing ever been attempted in any theme park?
I don’t know of any, but there may be some coasters with cars on gimbals.
The problem is that your body won’t be comfortable sustaining the forces imposed in a level position. The tracks are banked for a reason, and the seat supports are designed to keep the back and neck supported.
There are a lot of coaster now where you hang from a suspension, and those would fit the first criteria, when you’re moving up the first hill, you’re still pretty much horizontal. Forces being what they are I’m pretty sure you start to move parallel to the track and not the ground once you get a bit of speed going.
“X” at Six Flags Magic Mountain does something like what you’re describing. There are four seats, two on each side of each car, mounted on what’s basically a big axle. It isn’t free-rotating, it’s designed so that they do things like aim you face down when you go over a big drop (the track is in the center so there’s nothing under your feet). There’s no reason they couldn’t build it so that you stayed in a fixed orientation to the ground except of course that the ride would have to be planar - no banked turns for example. G forces when going through certain maneuvers might be funky as well but mechanically I don’t see any reason why it’s impossible.
Here’s a POV from “X”:
For some reason that reminds me of my first time having sex 
At the beginning of the video the train is being pulled up the first hill and the riders are facing backwards and up so they can’t see the top of the hill approaching. As the train hits the first drop the seats rotate so you’re suddenly facing straight down.
I guess they could build it but why? It sounds like it would be a very boring ride.
Going over hills would be like riding the merry-go-round and going through loops would be like riding the ferris wheel.
As mentioned, X (now X^2 since they put new trains and added music and other special effects) does what you describe. It’s called a 4-dimensional roller coaster because an additional rail on the track goes up and down and essentially controls a rack-and-pinion drive that enable the seats to pitch forward or back independently from what the rest of the track is doing. You can see an image of the train here: File:X2-newtrain.jpg - Wikipedia
When the ride first opened, it was a mechanical nightmare. Not too surprising since it was such a new technology for coasters. It seems only recently they ironed out most of the problems. While the idea you proposed is novel, it does seem relatively tame compared to what the real ride actually accomplishes (flipping people so they face straight down in the near-vertical drop, rotating riders through loops so they remain looking straight down at the ground).
Additionally, I forgot about another ride that may do something more similar to what you’re describing. You can see an image of it here: File:Kirnu linnanmäki.jpg - Wikipedia Instead of the seats being manually controlled to a certain angle, the seats appear to be free-rotating. Here’s an on-ride video of it: http://www.youtube.com/watch?v=AayL1zZ4U8w&feature=related
