[n_plus_1_acc]

They posdibly mean this:

> Journeys to or from Heathrow Airport are priced at a premium due to using the rail tunnel between the airport and Hayes & Harlington. That stretch of line is not part of the Network Rail system but owned by Heathrow Airport Holdings, who charge TfL an additional fee for each train that uses it. Heathrow is nevertheless included within the Travelcard scheme and daily/weekly fare capping as a fare zone 6 station.[80] https://en.m.wikipedia.org/wiki/Elizabeth_line