I took this photograph in 2001 at the former SP shops in Sacramento, CA. I believe it was being held for the California Railroad Museum.
This 40-foot Railway Post Office car was built in 1909 for SP's subsidiary Texas & New Oreleans. It was transferred to SP in 1951 and renumbered 6009. In 1956 it was reconfigured as a caboose and given the number 475. As a caboose, it served on the San Francisco peninsula. Those look like the trucks installed when the car was made into a caboose that operatedat least well into the 1960s.