Rail posts are very heavy duty fence posts that are made from rails that usually form train tracks. They are used for fencing in some parts of the world because they are very strong, made from very thick and heavy metal, and will take a very long time to corrode.
Fences that are made from rail posts typically also have very large concrete footings to hold the large, heavy fence posts, which makes the posts very sturdy.
Because these train track sections are very hard (if not impossible) to drill into, rail post stays are usually welded in place on site.
In many cases, if you want to remove a rail post fence, the easier way to do so is to cut the rail posts off at ground level using a cutting torch. While this does leave the concrete footings in place, it also eliminates a lot of the excavation and rubble removal that would be required to remove them completely.
This type of post is not used very frequently anymore, and you are more likely to have to remove one than install one.