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.