The rail line of a surfboard design is also called the plan shape or template shape. It is the curvature from the nose to the tail when looking at the surfboard from above. The design varies by the amount of curvature used to affect acceleration and maneuverability.
Curvier rail line - The rounder rail line allows you to bury the rails deeper into the water giving you more acceleration out of turns. A rounder rail line also makes it easier to change direction of the board because there is less side directional water resistance. An exaggeration of this concept provides a good mental image: If a surfboard is shaped completely in a circle it would be easy to change direction but difficult to keep straight.
Straighter rail line – A straighter rail line is often found in longboards or boards built for bigger waves or more speed. Straighter rails are useful for adding more surface area of the board to trim the face of the wave. Guns have a straighter rail line for added stability needed in high speeds on big waves. Conversely, this makes turning and changing directions more difficult. Straighter rail lines will cause the board to lose acceleration through turns but faster in straight lines. Imagine a stretch limousine taking a corner compared to an old Volkswagen bug. Which do you think will turn better and which is better suited for speed on highways?