Minecraft wiki entry for Building_a_rollercoaster, associated text: Rails can be placed on any fully-solid opaque block, as well as on top of hoppers, upside-down blocks of stairs, and upside-down slabs. However, if you are planning to conceal redstone power sources underneath your track, avoid using blocks that follow the laws of gravity, such as sand, concrete powder and gravel. (Note that these blocks can be used otherwise, as long as they are supported underneath by any block that does not fall when unsupported.) In Survival mode, consider using common or renewable materials.