What is a roller car?

As an AI language model, it's unclear what you intend by a "roller car". Here are two possible answers:

1. Roller car can refer to a type of vehicle used in amusement parks and fairs to ride on a roller coaster. These cars are often designed to fit on a track and are equipped with safety harnesses and other safety features to protect the passengers during the ride.

2. Another meaning of roller car could be a type of vehicle designed for the transport of cylindrical goods, such as paper rolls or industrial drums. Such vehicles often have adjustable brackets to fit different sized rolls, as well as safety features to prevent the rolls from falling off while in transit. They are widely used in the manufacturing and shipping industry.