Mud has collected and dried at the bottom of the curve. If the rust is only at the bottom, it’s easy enough to cut out a section of the “floor” then replace it with a plate welded in. Make sure you clean out all the crap, which is a tricky job thanks to the bracing reinforcements inside the rails. I had some success with a thin probe on a jetwash, which you can wriggle past some of the braces. Cut a couple of decent-sized drain holes in the new plate and be sure to prime, paint then Waxoyle the whole thing. The other rail is probably just as bad. Check all drain holes are clear especially after a very wet muddy day out and always jetwash the underneath of your Landy if you want it to last.
Our ex-MOD 90 had a chassis that was choked solid with compacted sand, it took me two days to clean it all out and some interesting plants grew on our gravel drive after that.