I have a similar 125lb Arm and Hammer, from Columbus, and its a great old anvil. You definitely do not want to attempt to remove the top plate- it is not "pretty easy" to replace one- these were originally either forge welded on, or cast in place, depending on the brand of anvil, and either process is virtually impossible to replicate in a home shop. The Fisher anvils from Trenton New Jersey used to cast a tool steel top plate into a cast iron base- basically, nobody today can do that- the theory is understood, but the institutional knowledge, and equipment, does not exist in one place any more. When people stick weld a plate on, by welding around the edges, you get no weld in the middle, so the top plate just bounces around and does not conduct the blow very well.
If you have worked with good anvils, you can immediately feel the difference when trying to use an anvil with a loose top plate. They suck.
Nowadays, labor is a more significant factor than the raw metal cost, so good anvils are usually monolithic castings of good hardenable steel.
My other anvil is a Nimba, made by my late friend Russ Jaque, in Port Townsend Washington. His wife still sells these, and they are incredible anvils. The design is based on a 19th century Italian anvil he used to have, and it is quite a bit more sensible than the classic "london pattern" most of us think of when we think anvil. It has more meat where you need it, meaning more of your energy goes to actually moving the metal. You can feel the difference between swinging the same hammer on the Arm and Hammer and the Nimba, on the same weight of hot steel- the metal moves more on the Nimba. For an old guy like me, less arm strain for the same result means I use the Nimba 9 out of 10 times, and they are right next to each other.
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Needless to say, not cheap. Made entirely in the USA, cast from 8640 alloy, heat treated, then hand ground and finished by one of the best blacksmiths in the world (Jim Garrett) who has an incredible eye, and LIKES grinding. These will last a hundred years, though, and are quite beautiful to boot.
The other main players in the anvil biz right now are the Czechs- an old soviet era foundry, Bronko, makes european style 2 horn anvils, and they sell them here under a couple of names, cheaper, but still not as cheap as an old one fresh from the barn-
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Railroad rail, by the way, makes a particularly poor anvil- the shape of the section means that the narrow neck of the vertical wiggles all over the place when you apply a lot of force. This is not a big deal with a train- its actually desirable to spread the load out. But when you are hitting hot iron, you want maximum bounce to the ounce- hence the very large mass down low of most commercial anvils. When they design big power hammers, they often go with 15,000 or 30,000 pounds of cast iron for the anvil, in big blocks- not tall skinny wiggly little things. Railroad anvils look cool, and are worth collecting as folk art. Some of them are really cleverly made. But for serious metal banging, a plain old cube of steel is far better than something that looks a lot like an anvil, but is wiggly as a worm. Knivemakers often just use 6" x 6" x 24" blocks, on end. Or even just a
3 foot piece of 4" round bar.