trip arm and then operate the train to a less restrictive signal. Where neither of these options exist, the only way to get past a defective signal is to "trip through". The operator at slow speed must trip the signal (which in turn trips the train and places it into emergency). The crew must then reset the emergency valve (by going out the front door of the train) before proceeding. Block signals Block signals are the most commonly used signals on the Toronto subway. They are used to keep trains properly spaced, and are controlled automatically by the trains themselves based on their distance relative to other trains, as determined by signal block occupancy. The following block signals are used by the TTC. Grade timing (GT) Grade timing (GT) is used to protect sections where a sharp turn requires a speed limit or where a downhill section would cause a train to accelerate to an unsafe speed if the driver were unwary. When entering a signal block which is subject to GT, one of two things controls the signal: the distance to the train ahead, or grade timing. If the current state of the signal is due to proximity of the train ahead, it behaves as a standard block signal. However, once the train ahead has travelled far enough for this signal to clear, the aspect does not immediately change but remains at red. When a train enters the block before the GT signal, a timer is started and the red aspect at the GT signal starts flashing. Once this timer reaches a predetermined time, the signal then clears as normal and the trip arm is lowered. If the train is travelling too fast, it will reach the GT signal before the timer expires and the trip arm will force the train to stop. The signal at the start of block before a GT signal has an additional white light (termed "lunar aspect" by the TTC) below the other aspects, which illuminates when the following GT signal is at red only due to grade timing. Where there are several consecutive GT signals, a lunar aspect will be present on the signal at the start of each timing block. A flashing red without the lunar aspect (not included in the images above) would be used for the signal at the end of the last block of a GT section. In addition