signals (and number of signals that are red when a particular block is occupied) is set in accordance with the stopping distance required by the trains. If a train violates a signal, the trip arm would trigger an emergency brake application to stop it before it reaches the train ahead. In other words, for a particular signal to clear, the system requires two or more full blocks ahead of that signal must be unoccupied. This requirement reduces the number of trains that can operate on the line compared to using automatic train control. Grade timing, a method of speed control, is integrated with the signalling system. Station timing signals, which allows trains to safely run closer together at slower speeds (such as near station stops), are also present. Headway control, a method of evening out the spacing between trains, is active at certain stations with interlocking (or home) signals. Such a signal turns red as a train passes it, and will remain red for a variable amount of time. This time depends on the distance between the last train that passed the signal, and the train that comes after the next train. This system is computerized, and can accurately calculate the relative distances between trains. If the next train is closer to the train before than the train after, then the signal will hold the train at the station. If the next train is closer to the train after it than the train before it, then the signal will clear. There are several limitations to this signalling system that can result in "signal problems" and "signal delays". One of the most common problems is track down. A track down occurs when a block gets a false reading and places signals into the danger position even when there is no train occupying the block. This can occur if debris interrupts the block by grounding out the track circuit mimicking the electric circuit caused by an actual train in the area. When a signal fails to clear, depending on the area, there are three different ways to rectify the situation. At some signals, transit control can perform a "call-on" where an orange aspect blinks and the trip arm is released even when the aspect displayed is red. The second option is a "key-by". Some signals have a plunger that the operator can stop, reach out the window, operate the plunger dropping the