to lunar white signals, grade timed sections are sometimes indicated by a sign with the letters "GT", or simply "T", in white. Station timing (ST) Station timing is used to reduce the separation distance between trains that are operating at slower speeds. It is typically used in and around station platforms, allowing a second train to safely move closer to an already-occupied platform than would otherwise be possible. As with GT signals, a timer is started when a train enters the block preceding the ST signal. If the timer expires before the train reaches the ST signal (i.e. the train is travelling slower than a defined speed), the number of blocks ahead of the ST signal that need to be unoccupied for the signal to clear is reduced. This reflects the shorter braking distance of a train travelling slower than the maximum permitted speed. Interlocking signals Interlocking signals are typically used in interlockings, which are any areas where train movements may conflict with each other. They are controlled by either human operators or a computer, not automatically by the movement of trains. Interlocking signals also tell train operators which way points are set. The following interlocking signals are used on the TTC. Interlocking signals can also include grade timing and station timing, which functions in the same way as for block signals. For GT interlocking signals, only the top red aspect will flash while the timer is running, the bottom red aspect remains constantly lit. Signal numbers All signals have an alpha-numeric number that relates to their location within the subway system. The number is assigned using the Chain system of measurement, whereby a signal's number is assigned based on the nearest chain measure. Each line or portion of a line has an assigned letter, and that precedes the number ascertained by the Chain measure. Signals that are on a northbound portion of track use the nearest even valued chain measure, where signals on a southbound portion of track use the nearest odd valued chain measure. Temporary signals In work zones, staff place yellow beacons on the track bed between the rails to inform train operators that a "slow order" is in effect; the first beacon is usually accompanied with a speed restriction sign indicating the speed limit for the affected area. A green beacon indicates the end of a work zone and allows operators to resume normal operation. In