I've done a few rails over the years. On "Huckbolts" you don't take those loose, just zip through them with a 5" grinder and a .045", (3/32nds) disc. Knock the slug out with a punch. On this job I'd look at vertically splitting and peeling off the outer rail about 18" each direction of the crack. Then I'd split and peel the center rail about 12" each side of the crack. Then "deep vee" both sides of the inner rail and using a flux cored, gas shielded wire and after a vertical down root pass, I'd vertical up weave a double pass on both sides, stress relieve the area, and grind it smooth on the surface that will mate against anything. I would then do the same on a replacement section for the center rail, (of course only on the outside), and once again the same on the final outer rail. As mentioned, you need a crossmember in there which is lacking. Color indicating crayons are a valuable tool when post treating weldments for proper metalurgical properties. A rosebud tip is a great tool to use for this. The cooling rate is controlled by tip distance. Practice makes perfect. Rob