Roller coasters use only a few different types of forces and energies.
The most important is the first hill of the coaster. As you go down the hill, the potiential energy changes into kinetic energy as you speed up, however some of the energy is absorbed by air resistance and the wheels turning. Since there is no other source of energy, the first hill is always the biggest. Also, the speed of the roller coaster is limited by that first hill. The higher the hill, the faster you will go.