I'll only speak to the GN side because I do not follow/study the WP
in any detail ...
It does not surprise me that the GN trains into Bieber were run as
extras. But the GN definitely was still running numbered freights into
the 60's - on other parts of the system. Such as over the Cascades
and the rest of the GN mainline between the Twin Cities and Seattle.
And the later in time you go the more likely it is that they were
run as extras.
The GN line from K Falls to Bieber was not a 'heavily trafficed'
line after some time in the 20's. One, maybe 2 trains a day and
for much of the time after 1935 or so was probably even less.
I do not have any precise knowledge of when things might have changed
to 'all' extras on the Bieber/Oregon parts of the GN. I don't even have
enough ETTs to answer the question of whether or not that particular
part of the GN was ever run under TT&TO ... but I would be very
surprised to learn that it never had TT authority operations.
So consider my response to be "a very fuzzy educated guess - with
a lot of missing details". Not the answer you were looking for - but
perhaps it helps?
If I had more ETTs I'd do the research - but in this case we are
left to waiting for someone with a more complete set.