Interesting observation. This gives us a one-year window for when WP tank cars were reclassified to MW service. It would be interesting to figure out approximately when the change took place. If I still lived on the Left Coast or had occasion to visit there, I could check the ORER issues in the CSRM library, as they have/had a nearly complete run from that time period in their reading room.

I've never figured out where the WP got their bunker-C fuel oil, but AFAIK they didn't serve any refineries directly, thus the need for the tank cars to be classed as revenue equipment for interchange purposes. Likely their cars went to a refinery in the Bay area on the ATSF or SP. Salt Lake City is unlikely, since the WP was coal-fired east of Winnemucca. Perhaps with the switch to diesels nearly complete by 1950, and the few remaining steam locomotives running on borrowed time or only fired up for the fall rush season, there was no longer any need to send their tank cars off-line. The entire system was completely diesel east of Oroville in 1950.  According to Dunscomb, in 1951 diesels handled 87.4% of freight train miles, 99.7% of passenger miles, and 82.5% of all yard switching hours. The last regular movement by steam was in June 1953 when mikado 329 ran between Oroville and Stockton.

Of course, as was typical of the WP, even though the tank cars were no longer in interchange service, many were not relettered. I still was seeing some of the 12K tanks in original lettering shortly before I left California on New Years Day 1982, though most had "MW 0" added before their car numbers.

Yours Aye,

Garth Groff  🦆