Route 78 hasn't changed a lot in the past generation; but when it was extended to Nunhead, it lost it's double decks in favour of plaxton and alx200 darts.
Now, it is being converted back to double deck, using E400 tridents from routes 38/242 .
It is really not very common for TfL to convert routes back to d/ds from s/d in the days of financial cuts and budget freeze!
you can actually find d/d on route 42 every day=- the s/d scanias are not exactly popular or reliable!
at least 3 PVLs at Q are usual performers on the route.