It's an express car - box motor originally built for SP service in the Portland area.
The SP Red Electrics were abandoned in the late 20s and many cars were sent to So CA. A few went to NWP and were used in passenger service- one got doors to become an NWP combine.PE modified mail cars, combos box motors and a few passenger cas. A coipla pass cars became deluxe parlor cars, a coupl more were converted to boxmotors, many more sat in the Columbia Steel Mill or the back tracks at Torrance and were scrapped just in time to become japanese navy ships or bombs or whatever [maybe] A couple of the cars that the NWP used were re-numbered briefly and used in SanJose-SF commute service, until sent to San Luis Obispo where they were converted to MoW service on the Coast Division. Visalia Electric got a Portland car and used it as caboose #25 , then it went to the ground at the Bakersfield roundhouse for the foreman's office. Portland cars were nominally 47 ft. The SP Oakland -Alameda -Berleey cars, and the later NWP cars of similar design were nominally 58 feet, but that doesn't count the double wide step vestibules. They were wider and longer than Harriman Standard passenger cars, hence the appellation [Helium filled Blimps were a common sight in LA area skies for anti-submarine patrols, and that name is from a WW1 british cartoon character]. SP had put up ownership of OAB cars as security for the cost of auto train stop equipment so they could operate over the Bridge Railway [on half the lower deck of the Bay Bridge]
so at abandonment the cars mostly went to the Toll bridge Authority, save one for a lunch room in oakland [and ??]. The United States MAritime Commission- the folks who ran the shipbuilding program, took most of them. A few of those went to Houston , maybe Portland - it gets murky here... Some combos and coaches went to Utah for service to the Arsenal ten to housing somewhere in more southern Utah, and thence to Western Railway Museum. Service for workers to the Terminal Island yards went via PE and boat, until USMC built track across the bridge to serve the yard directly, and put cars in service PE eventually leased and bought cars and after the war rebuilt them . The NWP cars were [ probably- gotta heck] actually owned by SP Company and were stored in the yard at Sausalito They went to PE directly. PE built boxmotors for use as locomotives. After a union agreement in __________ A boxmotor could haul one car- unless a full 5 man crew was used- actual hauling capacity was much more. In latter years PE stationed wood boxmotors at Glendale and San Fernando to spot cars interchanged from the SP
So, back to the original question OAB had two box motors. REgular service was storage mail from the Oakland Pier to Berkeley.PE widened the doors and used them for express service. One body went to the SP in Bakersfield as a shed into the 60s .
And last but not least were the OAB cars used on the UP and ATSF to haul servicemen for demobilization to Camp Hahn and San Diego. At least one of those cars was lettered for and used on the ATSF, the cars used on the UP went to PE for parts.
See the ERHA website, Ira Swett's books, etc., and visit OERM to see complete OAB, NWP, and Portland cars, as modified and used by PE.