Craig, I had considered that, but, dismissed it, mostly because I didnt give any thought to smaller rails. It didnt seem likely that someone had made it from regular train track. Good eye you have there.
I imagine you could use any kind of shot to dampen a mount, but, lead would be hard to beat. I figured it wouldnt rust, and I could buy it reclaimed from my local club, 1 mile north of me.
The vise is about 90 pounds, and Id hazard a guess the mount filled with lead shot and some oil approaches 400. The whole thing is bolted down with concrete anchors.

Id hate to back into it with the car.

Best,
Ted