You are gnat's crotchet away from the full answer.
19/10/65 was the last Royal Train out of Ballater, closest station to Balmoral, and hauled by D6142 and D6145. As you say, the Aberdeen-Ballater line closed in 1966.
27/01/66 was the last Royal Train out of Wolferton, closest station to Sandringham. The locos are not identified but train was booked a single Brush Type 2 to Kings Lynn and a pair from Kings Lynn to Liverpool Street. The Hunstanton branch closed in 1969 but was effectively out of bounds for loco hauled trains by Christmas 1966.
10/05/68 is another station, with big associations with Royalty going back to Queen Victoria, receiving its last Royal Train, this time an arrival not a departure. The line and station are still open, but something happened in 1968 so that it could no longer receive the Royal Train.