Conversation so far:
when does a roller coaster train have the highest potential energy
Gravitational potential energy is greatest at the highest point of a roller coaster and least at the lowest point.
what is the highest point of a roller coaster
The first hill of a roller coaster is always the highest point of the roller coaster because friction and drag immediately begin robbing the car of energy

Question: where do the cars move the fastest
Self-contained rewrite: where do cars in roller coasters move the fastest