Filling DIY RR Track Anvil with Sand?

[IrregularReno]

Good evening folks, over the past month I've been working on making me a new anvil. It is a piece of railroad track that I cut in 2, cut part of the wide base off, and welded side-to-side. Of course, everything was heavily beveled for good penetration and it seems very solid. However, with it still being railroad track, I have a hollow spot in the middle. It's about 2in wide, 4in tall, and a little over 8in long, therefore it would be very difficult to fill with any kind of metal at least for me. I will also be drilling a 5/8in hole near one end for a hardy/pritchel, pretty much so I don't have to switch to my other anvil if I'm punching or drifting a hole and straightening coil spring. My question is, if I filled it with sand, would it negatively affect the anvil at all? I understand that the more mass an anvil has, the better, but I was unsure if it might take away the little rebound that the anvil has.

Pic included.

[Purple Bullet]

Nice work. Sand shouldn't hurt the rebound any. If you want rebound you may need to harden the face. The rails can be heat treated but I don't know what the weld filler is. Way long ago I used some hard surface rods to repair the face of an anvil that I still use but the last time I priced hard surface rods I went into sticker shock. Another possibility for a softer section is something called "Cherry Red". If you go that route, be sure to read and follow the directions. It can make a thin layer hardenable.  

Of course the other thing you might do with the soft section is leave it soft. Its good to have a soft area for chiseling into. I doubt that right down the middle of the anvil is where you'd want it, though.  Still, I wouldn't obsess over it. The nice thing about anvils made of junk is that you don't mind grinding on them to dress them as needed.