Something I wrote for a Newbie's Blacksmithing forum.

I see a lot of questions and myths about anvils made from Railroad Track.
1) IMO they make awesome anvils and can be fabricated to YOUR specific size, weight and configuration.
2) All rails are not created equal and are often a proprietary recipe from the owner. Typically it is a medium carbon (0.80 max) and Manganese (1.20) steel.
3) Yes it can be quenched and tempered, but NOT recommended. By
Q&T, there is the possibility to make it too Brittle. Plus, it's a gonna takes you a one BIG azzed fire to do the job properly.
4) Unfortunately, they can be a labor of love and time consuming to build. But as the ol' Smitty says... "Make a tool to make a tool".
5) Yep, the bottom side is softer than the (worn) top side.

So, Rail steel is similar to T-1 and AR steels. They come from the mill Q&T'd. But the mill has chemists and knows what they are doing with zillions of $$$ worth of hi-tech equipment to do it with.
These manganese steel recipes are designed for impact and wear resistance. Motor grader cutting edges, drowser bars on the cheeks of dozers are also examples of really high strength/wear steels. These are different than what the buckets themselves are made of (T-1).
When rails come from the mill, they are differentially Q&T'd on the tops only. The bottom is much softer. After a bit of train traffic, the tops become much harder.
Now on to how to fab up an anvil.
A) Leave it as is, mount onto a log (maybe drive a few large nails or spikes to keep it in place) and start pounding away.
B) Weld the rounded parts with a medium soft hardfacing welding rod (stick or wire) and grind off until flat and smooth.
C) Attack it with an oxy-acetylene cutting torch. Carve a pointed horn, grind the hell out of it all to remove the torch cut nicks and grooves with the top flattened, scallop out the front and rear, then scallop out the base pate and you will have a reasonably respectable looking ASO (anvil shaped object).
D) Fabricate... Start off with a much longer piece of track than needed. Trim to basic overall length and shape a horn and scallop as recommended in step B. Now, with the left over length of track, cut off a small piece for a table and a larger piece for the face. Weld these in place as you see fit for your own personal design or as close to a traditional shape as you like. My preferred weld metals for this are plain old E7018, or if money is no object use E309-15 stainless steel stick rod. I recommend the 309 because of the high nickel will reduce the tendency for cracking. You can also use an equivalent Flux cored wire in the largest size your wire feeder can handle. Note: if you have a 110V unit, this will take a LONG time and will be very difficult to get a solid weld. Just saying...
E) Or any combination of the above.
OK now for the techno mumbo jumbo part, but I'll keep it simple, brief and low keyed.
It is true that the flat bottom is softer than the top. Top side is quench hardened at the mill, but is still not as hard as when after it has been in service. Manganese steels (some recipes receive little to NO benefit from attempting to quench harden due to low carbon content). With rail track having a medium (0.60- 0.82%) carbon content, attempts to quench harden either results in little to no gain OR too much and makes the face susceptible to cracking and chipping. Worst case scenario is that there could be a catastrophic failure (big crack!) and ruin all the hard work invested up to that point. As was stated earlier, different rails = different chemistry and I am trying to be as generic as possible.
Now that you have weld fabricated your dream anvil customized to your exact dimensions and shape, it is time to do one more thing... The bottom plate which has now become the top face to do all your work on is... wait for it... Too SOFT to do any serious work on. The manganese steels respond to work hardening. The problem with work hardening is that to do it yourself requires a lot of WORK.. Dad-gummed-danged-it. Too bad you cant just let the train reharden it for you. Soooo..... Rent, borrow or buy a chipping gun. A 10 pounder will be more than enough. A small pistol one will suffice.
Buy or make a flat blunt tool that resembles a planishing hammer and start pounding away on it. Then hammer away some more. Try to keep it even and overlap the impact marks. Flatten with a grinder and then pound again some more.

In about 6 weeks I'll be home and plan to make another one by fabrication as was detailed in "D" above. If so, I'll repost this with fotos. In the mean time, I will answer specific questions.
As long and drawn out as this was (I Hate typing!!!), to fully detail all involved would have been at least 4 times as long as it is already.
Hope this helps, but I'm sure it will raise plenty of questions.
Disclaimer... please follow any and all safety rules and precautions.
Also, I am NOT a professional smitty, but I do work with metals for a living... sometimes...
Foto of the railroad track anvil (60 pounder) was shaped and squared by carbon air arc gouging and flat profiled by TIG welding with harfacing rod and hand ground flat. Over 100 blades were built on that anvil along with numerous other art and industrial projects.