There was a 40 year age limit on freight cars in interchange, which changed to 50 years a few years ago if certain conditions were met.

I know many of the newer SP chip cars went to Georgia Pacific with GPSX reporting marks, plus some for other railroads. Some of these were still in wood chip service between G-P mill in Toledo, OR and Collins Pine in Lakeview, OR - at least a couple years ago. The paper plant at Wallula still gets some chip cars off the BNSF (end dump only?), never noticed any via UP.

I've also worked on a shortline which received cotton seed for feed in chip cars. Kind of scary how they unloaded them. They put a temporary dock at the end of the car, propped the end door of the car up and ran a skid steer in and out.

Realize that cars built before 1971 are now under the 50 year rule, which is not only an AAR interchange rule but a FRA safety regulation. To continue using a 50+ year old car in revenue service you have to apply for a FRA waiver. For continued maintenance of way use on the owning railroad, no restriction - but what would a chip car be used for?