You sure it's going to be the back plate? It would be a simple matter to hinge the plate, to fold down when pulled by a string, and then a spring can pull it back up after. To work both plates would be a bit more difficult unless one can put an electrical motor on. Problem is to remember to do it at every gantry gate. I will put my, not engineering but scheming, mind to work on this very important matter. The nation, or more precisely, my wallet depends on it.