signaling, Union Pacific and BNSF use speed enhanced route signaling (or what by now is effectively speed signaling with some route elements), and Norfolk Southern uses a mix of speed and route signaling based on the original owner of the line. Commuter railroads and Amtrak all use speed signaling where they own or maintain the tracks they run on. Canadian railroads all use a strong system of speed signaling in Canada, but have some segments of route signaling on lines they have acquired in the United States. Common signaling practices Signal types North American signals are commonly of three types. Absolute - Absolute signals are usually connected to an interlocking controlled by a block operator or train dispatcher. Their most restrictive aspect is "Stop" and trains cannot pass them at Stop unless they obtain special authority. Absolute signals will default to displaying Stop unless expressly cleared by a control authority. In older practice, multiple signal heads are directly above and below each other on the mast. Automatic - Automatic signals are governed by logic connected through electrical track circuits which detect the presence of trains or obstructions automatically. Automatic signals are permissive with their most restrictive aspect being one of the "Restricted Proceed" variety. Trains can pass an automatic signal displaying "Restricted Proceed" without any outside permission. Automatic signals are typically recognized by having an attached number plate and in older practice, having multiple signal heads offset from each other on the mast (i.e., on opposite sides of the mast). Semi-Automatic - Semi-Automatic signals are those that typically act as an automatic signal, but can be set to display an absolute "Stop" aspect. Semi-Automatic signals do not have a number plate, but can display an explicit "Restricted Proceed"-type signal. Other types of signals can include Train Order signals, manual block signals or signals governing special safety appliances such as slide fences, non-interlocked sidings, road crossings, etc. These are much less common than the three standard types. Layout North American signals generally follow a common layout. A high signal consists of one to three heads mounted roughly in a vertical stack, each head capable of displaying one to four different aspects. Automatic signals are identified with a number plate whereas absolute signals are not. The signal's aspect is based on a combination of the aspects each individual head displays. Where a signal has multiple heads, aspects are read from top to