http://assets.dft.gov.uk/publicatio...ailways/chiltern-franchise-agreement-2012.pdfI don't believe they are required as part of their franchise to operate a service to Paddington.
DfT said:Schedule 2 —
Franchise Services (Clause 26 and others)
Part 1 — Passenger Services 44
Railway passenger services (excluding any Charter Services) in each direction via the
following routes (and, in the event of a disruption, any reasonable diversionary route):
....
(4) Paddington to South Ruislip via Old Oak Common West Junction, Greenford East Junction and Greenford West Junction;
....