I could be wrong, but I suspect the anti-rattle springs that clip to the pads helps prevent these low spots wearing into the brake rails. My XJ didn't have the anti-rattle springs on the pads when I bought it, and had deep wear in the rails. Arguably NOT lubing the rails and pads during install with disc brake grease was also a factor. (The anti-rattle springs are those little wrap around springs that go over the slot in the pad backing. That will hold the pad off the rail, and take up any slack so the pad can't bounce up and down with the open space between the pad and two rails, unless of course you're actually braking which then they'll compress and allow the pad backing to press firmly on the rail.)
Oh, and the deep wear spots in the brake rails, combined with worn out shocks and front control arm bushings resulted in death wobble on my XJ when hitting the brakes at high way speed when I first purchased it. Fixing all three of those has resulting in never experiencing it again the last 10 years.
I've done mine also, I did post a pictorial of it when I did it, like 10 years ago. Heck, I may be remembering wrong and it might be on a totally different online forum.
Keep in mind a professional welder will tell you welding cast iron is very specific and has all sorts of special procedures an amateur would never be able to do. BUT, this is for structural strength of the entire cast iron piece, NOT filling a wear spot. I.E. if your steering knuckle was broken in half, I'd say your crazy if you think you can weld it together and think it was safe to drive on, but that's NOT what you're doing. I and others have proven, filling in the wear spot with weld metals works and lasts.
I'm less knowledgeable on welding than 8Mud. In fact I'm very amateurish. Welding with a Mig welder is NOT so difficult that most mechanically inclined amateurs can teach themselves how to do it (with some reading). But it is difficult enough, that if you think you can just buy a cheap used Mig machine, fire it up and weld in that spot with no practice or learning, you're sorely mistaken.
When I fixed up my XJ, I had friend lend me his Mig Machine. I read and practiced several days on scrap metal, before I started welding. First I did all the rust repair on the floor pans, which was a lot and got my skill down, then attempted the brake rails.
In my case, I can't remember the specific details like 8Mud, but I remember some details on my approach that seemed to work.
*thoroughly cleaning the worn spots and surrounding area, like grinding it, power wire brush to take a little metal off the top, so I could get the purist weld with most consistent metal without impurities.
*I turned the Mig Welder up high, made a nice big puddle of molten metal and laid it on thick.
*Since I had to do 4 spots for each steering knuckle, I alternated steering knuckles and even waited in between, to make sure each steering knuckle was cool to the touch before starting the next weld spot. Not only do you want to avoid getting the cast iron too hot, effecting the metallurgy, but also if the steering knuckles are still on the vehicle, wheel bearings, drive shafts, u-joints, ball joints and grease boots can be damaged by the heat as well.
This resulted in a huge glob of metal over the worn down spot, I used a angle grinder to get it down close, a dremel to get it down even closer and then finished it off with a file to get it perfectly flat.
Take the advice of the more experienced welder if they tell you the better approach is to lay down smaller multiple beads, to build up the metal, thoroughly cleaning and allowing it to cool, in between each bead. In my case it worked fine and has lasted, by laying down one huge bead, a big freaking puddle, into the spot all at once, then letting it cool off a long time before going to the next spot.
And then, once you've fixed it, make sure to get the anti-rattle clips for the brake pads and use specific brake grease for the rail/pad interface. Other greases won't stand up to the heat or wash away easily when exposed to the elements.