A while back I was looking into getting an anvil and trying to learn about them etc. There is a great video somewhere, in which this crusty old blacksmith lays it out pretty well. All you "need" is MASS under the hammer blow and it doesn't need to be a "real" anvil. He didn't care much for railroad track because in his head, the narrow center reduced the effective mass under the hammer strikes. His claim was that the sides were not of much use. A better alternative was to go find as large of a block of steel as you could. He described an example that was something on the order of 4x12x18. He had it standing on end so the 4x12 was the striking area.
I'm not a blacksmith in any way shape or form, but what he said made a lot of sense and it might be easier to find a large chunk of steel than a real anvil. Of course you won't have a horn unless you add one but it's a good start.
A wild example was a big old diesel cyl head I saw used in a pinch.
