You noticed this had shots that had both road trains with, and without cabooses. And, the local still had one. This date is pretty late in the period when caboose's were being phased out on the SP, that had started way before I left. You'll have to get an ATSF guy to answer why some through manifests like those trains still had cabooses, and some didn't?

Maybe, they didn't have a working rear end device? Today's devices are more reliable. There were early "teething" issues with keeping them working, often battery life, getting mangled or trashed, or harsh weather condition issues, and a even few "incidents", due to these occasional reliability issues, as I seem to recall.