Dont pour the concrete. If you pour concrete over them, then it will be permanent. You have the potential to have a really nice crawler course at someone's place I presume is near your home. Take full advantage of it. You will need to get pretty good size rocks. You'll want it to be like a mound, so if you can't get enough rocks you may want to use like some milk crates, big chunks of wood, etc... to build the middle up higher than the rest. As you start laying the rocks out, just kinda see where you start to see some lines forming to go up on one side and back down on the other, and go from there. I think a good place to crawl like that needs to have some different sides that you can crawl on so you don't just have on long line, but that's just me. That way you can crawl a bunch of lines without changing up the configuration. I think the concrete is a really bad idea, because as you start crawling you will notice gaps that you'll need to fill in, then you'll find one spot too easy and need to take something out, etc... As long as it's not in concrete, you can make the adjustments as needed. The railroad ties are a great idea by the way.
