On PFE cars the salt usually went in the ice bunkers. Industrial grade salt was used to speed evaporation of the ice and lower the temperature in the car. There were specific volume of salt based on the destination, route, commodity and weather to maintain the desired car temperature. The blocks of ice were broken up by an ice man with a long steel breaker bar and then the salt added. Seldom modeled are drain pipes in each corner under the car for melt water. Trucks would be splashed with the salty melt runoff when moved at speed.
Salting if required was often done at an icing platform when the car was pre-iced before moving to the shipper's loading facility. Shippers wanted their cars pre-cooled before loading. The ice/salt mixture would be noted on the movement waybill and often chalked on the car in case the car was re-iced en-route (often at Ogden, and/or Omaha: East St. Louis if coming off the Cotton Belt.)
Also note (from Tony Thompson's PFE book) that PFE washed cars nearly every return trip as cars were returned west to Roseville, Nampa, Tucson and other PFE facilities. This washing was discontinued about 1953 as car men's labor became too costly. Cars were hand washed on the sides with long handled mops. The ends and roofs were rarely washed. Note that there were very few westbound loads. Nearly all ice cars were returned west as empty. In heavy produce seasons PFE (UP and SP) wanted their cars back as soon as possible and ready to make another high revenue loaded trip east.
Your Trix car appears to be post 1953. Heavier weathering would be appropriate by the mid 1950's as PFE/SP/UP no longer cared as much about their image. The more shippers consolidated, the less they needed to be impressed with a clean car.