When did the UP introduce the roller bearing equipped UP "Dispatch" stock cars into service? ...I wonder if the UP was reloading stock into these cars at this time to be able to run the trains faster -- I seem to recall these special trains were able to skip at least one rest stop, perhaps Las Vegas?
Frank provided you with the info on the cars, I'll add a few comments related to their operation. I don't believe the roller bearing cars materially impacted train speeds vs. use of conventional friction bearing cars. Even with the addition of the 100s of "Dispatch" cars in the late '40s, note that shipments of livestock on the LA&SL trains were still moved using a mix of livestock car types, with a significant % being friction bearing cars from UP and from foreign roads. The promotional photos along the LS&SL at the time that (seem to) depict solid trains of new yellow "Dispatch" cars are deceiving in that respect. They were still running many brown UP cars along with cars from NP, CNW, CB&Q and others on those trains. So the mix of cartypes on those trains (friction bearing stock cars + cars often added to the trains as "fill") would have limited train speeds even if the new "Dispatch" cars were able to run faster…
The main advantage of roller bearings in those early years was that they greatly improved reliability of shipments - fewer en-route delays due to hotboxes, etc. Hotboxes were a real problem for RRs in the steam era. On the UP, I've noticed something on the order of one hot-box set out (en-route) per subdivision per day on average. As you may imagine, it was much more problematic to deal with a bad-order livestock shipment vs. the typical carload of freight. This was especially true when a hotbox or other problem made it necessary to set out a livestock car at some remote station in southern Nevada where the temps are normally in the range of hot-to-unbearable.
It's my understanding that the principal factor that made it possible for UP to eliminate the Las Vegas feed/water/rest stop was the installation of CTC. As wartime traffic dropped off, the additional capacity & flexibility provided by the CTC installation (vs. the TT&TO operation they'd been using) made it possible for them to run the DLS trains from Ogden/SLC - LA run in under 36 hours. Operation with diesels also began around that time, which further improved running times and performance vs. schedules.
So it wasn't so much the cars, but rather the way that they operated their trains that allowed them to eliminate the intermediate stop for livestock in the late '40s.