The Office of Rail and Road’s (ORR’s) Estimates of Station Usage statistics provide an estimate of the number of people entering, exiting and interchanging at the station. Entries and exits are estimated using ticket sales data, using the origin and destination of the ticket.
As Great Britain does not have a fully gated rail network, a complete recording of passenger flows is not possible and as a result, it is not possible to tell whether a passenger has used their purchased ticket or not. Therefore, in the case of cancelled trains, ORR assume that the passenger has still used their ticket, potentially on a later service or via a different route.
More information about how these statistics are compiled is available in the Estimates of Station Usage 2014-15 Methodological Report on the Office of Rail and Road website at http://orr.gov.uk/statistics/published-stats/station-usage-estimates.