translate([0,ym,0]) mirror([0,m,0]) translate([0,-ym,0])
translate([-1,-1, psz[2]-rail_zsz])
cube([psz[0]+1, rail_ysz+1, rail_zsz+1]);
translate([0,ym,0]) mirror([0,m,0]) translate([0,-ym,0])
translate([-1,-1, psz[2]-rail_zsz])
cube([psz[0]+1, rail_ysz+1, rail_zsz+1]);