I have been commuting on the High Speed HS1 service to and from Liverpool Street since 2009 when the service was introduced. I have a train only gold card season ticket to London Terminals with High Speed. And I have been able to travel to and from Liverpool Street on National Express with no problem. However, I was stopped his morning at the new barriers in to Stratford Regional at Westfield. Person at the barriers claimed that my ticket wasnt valid to travel to Liverpool Street on National Express. After making inquires, she found out it was as long as you have a gold card season ticket with High Speed. This validity is apparently due to an agreement between National Express and Southeastern.