It is only a one day travelcard. You can't buy to a boundary anyway can you? The ticket office will be closed anyway so it needs to be something from a machine.
I assume this combination of tickets is cheaper than an outboundary Travelcard from Tring?
The appropriate ticket would be a single to Boundary Zone 6. Somebody else will be able to confirm the price. If the booking office is closed and the TVM can't issue a ticket to Boundary Zone 6, you should pay for a ticket from the TVM with a value as close to the BZ6 ticket as possible (preferably in the right direction). Then excess the ticket at the first opportunity (this may be on the train or at your destination station).I usually buy a 16-25 travel card at Chalk Farm but use it on the bus to Swiss Cottage. From there I buy a Boundary 6 - Aylesbury and travel via Harrow on the hill. On the way home though I can get a lift to Tring which is far quicker. So I just need to know if I can buy to Hatch End or need to Harrow & Wealdstone to then complete my journey back to Chalk Farm on my travel card.
The ticket office at Tring isn't open so has to be something from the machine.
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The train does stop at Harrow and Wealdstone. It's only about a quid difference so I don't mind really, just out of interest!