Frank, I have documentation from NP records regarding fish shipments you may be interested in seeing. Send me your e-mail account off line and we can go from there.
Some of the comments I have seen:
1912-1930 Comment - Only older refrigerator cars should be used
for fish loading because once they are used in fish service,
these cars are unsuitable for anything else. - Yes, I know you
are looking for 1952, I am not sure when or if the above might
have changed.

I do have some documentation of Pacific fish (Seattle-Portland) being transported to several destinations, Chicago and east as far
as Atlantic coast destinations in these records.
Many of these are in or very near to your desired time frame.
Car numbers available in some cases. I will also try to get this
and other data onto this site (in my lifetime, if possible).
Jim Dick - St. Paul
.......................
I'd assume that fish would have been moved by reefer; I'm curious
as to whose reefers would be most likely to have transported said fish around 1952?