Hi Colin,
My understanding is that you can’t add credit to a 60+ Oyster card. I can’t personally verify this yet. I will flag up that you think the system may be allowing it.
If you touch in at Shenfield and travel to Stratford as an example, the system would need to use credit if your train was Greater Anglia, but not if it was an Elizabeth line service. The system has no way of knowing.
In this instance I think extension possibly relates to boundary of zone 6 tickets.
