Rather than put yourself through that extra flight and extra train, there may be another choice. Your 2 tickets must be cross-referenced with the number of your trans-Atlantic ticket printed on your airpass ticket, but that is all. Bearing in mind that you could, in theory, change the return date of your trans-Atlantic ticket if you were to pay a change fee, okay? Get this concept in mind. Let us say that you are thinking of postponing your return, but you just don't know the date yet. On that basis, add a third sector to your airpass as a throwaway, making it on a very short flight costing you the least money.

BA used to do it all the time. Their flights would always go out of London Heathrow and back, so say that you wanted to use their airpass to go to Athens and back. You would still need a third sector. So BA would sell you a London-Manchester sector as a throwaway. Usually you don't need to put a date on any flights after the first one - it just has to be a stopover.