Top Thrill Dragster
Sometimes, a train does not have enough speed to go over the  hill. This is known as a rollback, and it usually happens during cool, wet, or breezy weather. It can also happen when the wind is blowing against the train. In these conditions, the launch doesn't give the train enough speed to go over the hill, which causes it to stop before reaching the top of the hill and then roll back down the hill, which is why it is called a rollback.