A roller coaster typically begins with a chain and motor exerting a force on the cars to lift the train to the top of the first hill of the ride, which is also the tallest. Once the train makes it to the top and is pushed over the top of the hill, gravity takes over and it becomes an experience of energy transformation.
At the top of the hill, the cars possess a large sum of potential energy. That potential energy is equal to the mass and height of that object. After the first drop the cars lose a lot of this potential energy because of the loss of height, but they gain Kinetic energy, the energy of motion. Kinetic energy is equal to the mass and velocity of the object. So throughout the ride the initial Potential energy is just lost then gained, lost then gained until the end of the ride.
Below is the worlds tallest roller coaster, The King da-ka, located at Six Flags Great Adventure in NJ. With a height of 139m. At launch you are traveling at 206km/h. Only 10 Km/h less than a Cessna 182, a single propeller airplane.