The could be very old passenger car bodies set down on the ground to provide housing for SP employees. These could be cars originally built during the 1870's (or earlier - see below) and then down graded several times -- ending up in MW service and then set down on the ground. I have seen photos of similar cars set down on the ground.
Now and then these still turn up and some have been preserved. See this one preserved by the Northwestern Pacific:
http://www.nwprrhs.org/coach-29.html
History about 2/3 way down the page. Also check out link to photos. And also the archive pages.
Brian Norden