Railroad rails are roller forged, and have been for a long time. Cast iron or steel is too brittle to do the job. That’s why people are using it for anvils. Apparently US rail is fairly standard at ~50lbs/foot, and 12" seems to be the default length on Ebay.
Part of the reason for spot checking Ebay is it’s pretty loaded with small businesses and hobbyists selling more manageable chunks of material. It’s a common enough place to grab metal stock, specialty woods, small sections of fabric. Or things that are a bit harder to find. Ebay and Etsy are one of the few places to (probably illegally) get Blackthorn in the US as an example.
Though obviously if you order from a random seller in an out of the way place you may be getting something mislabeled or fake.
I think the bigger issue is likely to be shipping. A 50lb chunk of steel is probably not going to go out in a flat rate priority mail box.
