I'm not certain - which is why I inserted "I guess" - but having
looked at the Ian Allan Pre-grouping Atlas, the line coming in
from Aylesbury and Quainton Road seems most likely. (The A-Z
of Buckinghamshire is totally uninformative on this point)

I was driving from Winslow along Verney Road. A railway
embankment came in on my right. Shortly before Verney
Junction the road goes under an old railway bridge. Now,
assuming the railway to my right is the trackbed of the old
route from Winslow and Bletchley, the bridge brought in the
line from Quainton Road. What I need is a 1930s Ordnance
Survey map of the area!