Re: Photo: SP Flat Car(s) Transformer(s)
I'm guessing these are GE transformers, manufactured in Oakland.
General Electric had a transformer plant in Oakland, CA starting in 1924. (5441 East 14th/ International Blvd).
See "Project Background information" here.
Westinghouse Electric had a plant in Emeryville, CA from 1924, but it was apparently only building insulators, and later repaired transformers.
Westinghouse did build transformers in Sunnyvale, CA, but only starting around 1946 when they bought Hendy Iron Works. I did a quick search and couldn't find other mentions of Westinghouse manufacturing transformers in California.