The reading I have done about fish coming from Seattle on the NP indicates that a lot of it went as fresh fish in express refrigerator cars or baggage cars equipped with fish racks. I assume that the fish would be packed in in ice if in baggage cars, and those NP fish rack equipped baggage cars lasted will into the 1960s.
Freezing food didn't start happening in a big way until the early to mid-1950s, and I believe it was mostly frozen vegetables at the start. The first mechanical reefers were built for/by FGEX in 1949 (probably a few experimental cars, but I'm not sure) and 1951 (141 production 50' cars). PFE's first 50' mechanicals came in 1953. (This information was gleaned from RealSTMFC messages from years 2009-2010.)
Other people will probably chime in with more/better information.