You never really know what railroad ties are made out of, so you might get a quality wood, or it could be something horrible. Railroads put them on large crushed rock so they are never sitting in water, just laying on top of the rock. This allows them to dry out quickly when it rains and to last longer. Here in East Texas, with red clay, termites and just about every wood eating type of bug that there is, the do not last very long. The bugs eat out the wood on the inside, so they look fine for years, and then one day, they just collapse.
PT wood is better, but for what you are doing, I wouldn't use them. You are just creating a home for rodents. I've never seen a shed that was mounted on skids that didn't have dead rats under it.
I would have gravel delivered and then spread it out. For your container, I would think two inches should be find. You just want to keep the container off of the soil if you can.