A movable seat and a rail device. The movable seat includes a main body; an air passage, an air sucking passage and an air intake passage formed in the main body; a sucker disposed under bottom face of the main body and an operation/control member mounted on the main body. When the operation/control member is positioned in a locking position, the air passage or the air intake passage keeps in a communication state and make the sucker a sucking force. When the operation/control member is positioned in a releasing position, the air passage or the air intake passage is blocked. By means of easy operation, the movable seat can create sucking force and locate or stop providing sucking force. One or more movable seats can be connected with multiple beams to form the rail device, which can be quickly located or moved.