I've always loved riding roller coasters. And Minecraft is very limited in that you can only make the steepest drop a 45 degree angle. So making a roller coaster with a 45 degree drop at its steepest is kind of lame, but since there are no mods that I've seen that do this, I would really like it and appreciate it if someone could make a 'roller coaster' mod in Minecraft. I would clap if someone could make this. But, I bet many people have said this and I think there might be a reason why it's impossible for this to happen. Anyway, enjoy your new wall of text.
Hope I'm not too late after 8 years, there is the EX Rollercoaster mod, it may not be what you're looking for but I suggest giving it a try
I'm surprised it let me reply to such an old post