Minecraft wiki entry for Activator_Rail, associated text: An activator rail cannot be attached to the side or bottom of any block, but attempting to make such an attachment may cause the activator rail to attach to the top of a block under the destination space. For example, if a fence is on the ground, attempting to attach an activator rail to the side of the fence causes the activator rail to be attached to the top of the ground next to the fence instead.