Minecraft wiki entry for Powered_Rail, associated text: A powered 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 powered rail
powered redstone dust configured to point at the powered rail; a powered rail is not activated by adjacent powered redstone dust that is a directionless "dot" or configured to point in another direction.