Some have a return spring and some have a "spring sort of thing" that prevents the rail top from hitting the back stop. If you have your rail pounded into the bench, it's seldom a solid fit in the holes you make. One time, in my history of rail gun shooting, did I encounter a bench top that held the rail solidly in both directions. Anyway, you pound the rail points into the bench, and then bump the rail forward, as you look at the target through the scope, until it quits moving...completely. You will have to pick up the base and blow the dust out of the holes you made very often so just blow them out as a standard. OK, you're set up to shoot but if the rail ever hits the back stop you've got to re-set it, or re-aim it every shot.
Thus, the spring cushion thing (or bunjee cord) you see on many rail guns. Remember this if you don't have a rail gun - rail guns don't shoot better than bag guns, they just shoot faster. Given two guns, a rail and a bag gun, that have the same accuracy level, the rail will win over time purely because of the speed.