That is a tough question to answer. How do you know for sure? Well, I can tell you honestly that when I decided to go into engineering, I wasnt 100% sure it was right. Over the past 12 years I have just come to realize that it is right for me. It sounds like you have an excellent start by just thinking about this and finding an interest in roller coasters. If youve done really well in your engineering class and you enjoy and are good at math and science, then I believe that engineering could be for you. You will certainly enjoy things that come easy for you. Now, as for the roller coaster designI do not know anyone personally that does this, but several of my friends were interested in this in college. You would need to specialize in Civil Engineering structural design for engineering the track, but this type of design also requires a thorough knowledge of physics. So to be a roller coaster designer, would take a combination of creativity/3D visualization skills, Structural Engineering, and Physics.
Good luck with your tough decision. You will never go wrong with an engineering degree!