Rail on the surfboard is the curved rim of your board; rolled from deck to bottom and nose to tail (Rails are the edges of the surfboard where the deck meets the bottom).
The rail interacts with the wave water at its most sensitive point, as the water enters, flows along and exits the board’s rocker and outline. The shape of the rails controls the way in which the surfboard responds during planning and turning. Height of the rail determines the surfboards buoyancy and response.
Before choosing a rail height, think about how much buoyancy you require and your surfing style. In the middle of the surfboard the rails are larger with more roundness so they won’t catch or dig into the wave.
A general surfboard design rule when it comes to surfboard rails is the fuller and boxier the rails the more flotation and the harder turn the surfboard will be to turn, while thinner tapered rails will be less floaty and easier to sink into the water
Rail volume is one of the most important features of the surfboard. Hard rails will make the surfboard generally easier to turn depending on where the hard edge blends into the rest of the rail. While soft rails are for a more laid back style of riding.
Harder rail edges forward of the front fins will generally make the surfboard track more in one direction.
If you make a hard turn at the bottom of a wave and you have hard rails at the tail then the surfboard will tend to grip more to the face of the wave, without sliding.
If you want a surfboard that will make fast turns and quick wave release then you need a hard rail from the tail up to the back of the front fins.
How to choose the rail height?
The options are: Low, Mid low, Normal, Mid high and high.
Rail Height depends on the kind of waves you will be surfing and the style of surfboard e.g. shortboard, fish, funboard etc.
Generally the lower you go on rail height the less buoyant the surfboard will be and would be suited more for an advanced surfer who is selective about their wave choices. High rails are more suited toward beginner surfers however pro surfers will usually have a higher rail board for smaller sloppier conditions.