Mike, rail is not just one color, so you'll have to pick what looks best to you. Potting soil was just the name of the color I used. I got it at Menards, which won't help you. I think it was a Dutch Boy color.
Here are some pictures I grabbed off the net. As you can see there is quite a bit of variation.
This one is pretty close to what I used.
Here's one in good sunlight, it looks lighter.
This small chunk looks different still.
Here's a more rusty one. I don't think you should go this route, too orange, and it looks "cartoonish" on a layout.