The rail speed controller is used to manipulate the speed of your rails. Place it down and connect from it to all the rails that should be controlled. Using shift+V in build mode will add all touching rail blocks in the same direction. Then connect any number of activation blocks from the rail speed controller. The number of active blocks in opposition to the number of inactive blocks determines the speed percentage. So 5 active and 5 inactive is 50% of max speed, the same as 100 active and 100 inactive