On the Southeastern website, it seems quite clear that you need to buy a supplement for their high speed trains:
You can buy a supplement and upgrade to high speed services if you have:
- A valid ticket or Season Ticket for any Southeastern service into London
However, this isn't mentioned in the rail planner app, the seat reservations page, the country page or the supplements page.
Am I correct in thinking that this information is missing in all these places? Or is there somehow an exception for pass holders that isn't mentioned on Southeastern website?