The rails are the "edge" of the surfboard and run from the tail to the nose of the board. Rails are where the deck and the bottom meet. Like the other parts of the surfboard, rails have their part to play in shaping the overall performance of the surfboard. The shape of the rails determines how water flows over them when the board is planing and turning. Different shapes have different uses. Rails are thickest towards the center of the board and thinnest at the tail and nose.
Hard rails and soft rails are the two main types of rails. Soft rails are nice and rounded with no defined edges and a smooth transition. Hard rails have a distinct edge and may meet the bottom of the board in a corner. Soft rails are common on traditional longboards and provide good stability and plenty of drive. However, a board with soft rails will not turn as easily as a hard-railed board. The harder the rail, the quicker and tighter the board will turn.
Rails can be described not only in general terms like hard and soft but also in more abstract terms like characteristics of shape and proportionality as a ratio of distance of the widest point between the deck and the bottom of the board.
The Curved Rail Narrowing Towards the Nose