For the headway at a particular signal, the start of the headway distance is measured from the sighting point of the first signal showing a restrictive (non-green) aspect to a following train, if the preceding train was still occupying the overlap distance of the signal being measured. The end of the headway distance is the front of the first train when it is just emerging from the overlap distance beyond the signal being measured. The number of signal spacings between the first and second trains depends on the aspects which are capable of being shown: in 4-aspect signalling it is three signal spacings; in 3-aspect, two signal spacings; and in 2-aspect (rapid transit), from the first repeater to the stop signal of the next signal section beyond it.
So, for 42 train-pairs per hour, as on some Moscow Metro lines:
A high level of service is indicated a high number of train-pairs per hour or a short headway.
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