3600 divided by time between dispatchs (in seconds) gives you dispatches per hour.
Dispatches per hour times number of seats per train gives you ideal capacity. Real world numbers are unpredictable, but always less, due to empty seats, etc.
As an example say a train dispatches every 90 seconds (NOTE: ACTUAL DISPATH INTERVAL, NOT STATION SETTINGS) and seats 28 people per train.
3600 / 90 == 40 dispatches per hour
40 * 28 = 1120 pph