Minecraft wiki entry for Activator_Rail, associated text: An activator rail is a redstone mechanism and can be activated by:
an adjacent active power component (for example, a redstone torch, a block of redstone, a daylight sensor, etc.)
an adjacent powered block (strongly-powered or weakly-powered)
a powered redstone comparator or redstone repeater facing the activator rail
powered redstone dust configured to point at the activator rail, or configured as a plus sign.‌ An activator rail is not activated by adjacent powered redstone dust that is configured to point in another direction.