sounds much like a train system to me, only faster.
I definately think the problem of urban transportation needs to be solved, but I have not a big clue about how to do it. I can't quite relate to the word rollercoaster, since they're mostly driven by the pull of gravity, and i couldn't really imagine a big hill somewhere in a city where every transportationdevice in the city had to climb up to gain speed.
You could use boosters of course, but the result would be trains boosted from different locations, running at different speeds and hitting each other. If there isn't a seperate track for each train or cart, that is.
If you're thinking about a train either equipped with an engine, or constantly kept in speed by outside forces, then I'm much more positive to the idea.
The problem here is that either we have to have predifined routes much like trains and busses today, or else we need to design a system of tracks that are customizable, so you have control over where you're going.
While the last system is by far the best, it's also very difficult to make, I think, because you would need tracks to everywhere, and you would need to navigate each train-thingie through a dense traffic, changing either the tracks as you go, or somehow steering the train.
so... to sum it up... I like the idea, (if I understand it), but i think it's impossible to make right away. In time though, we'll probably see something like it. Or personal teleports like in star trek
