Ted, your anvil is the classic train track shop made anvil. The smaller size would have me guessing that it was from a smaller track like found in mines for the carts, or maybe overhead tracks for something like commercial sliding doors or some other factory overhead material handling system.

My 'anvil' is a 5+ by 5+ inch hardened post that's around two hundred pounds by itself. I made a steel base for it that has six bags of shot in it, and it's drawn tight together as one unit with chain. All together, it's a little over four hundred pounds. Nice rebound, no hopping around or vibration, small foot print.