Turntables are used by railroads to turn locomotives and cars around. There are only three in San Diego County that I know of. This one, one in Poway at the Old Poway Park , and one in Campo at the Pacific Southwest Railway Museum. According to their web page the one in Campo has not been assembled since they acquired it, though.
One rainy day in the fall of 1967 I had the good fortune to be present when the Railway Museum's Shay locomotive (which was stored on a siding not far from here) needed to be turned. I got to ride in the cab of the Shay while it was towed by a diesel (even though the steam was up, some silly safety regs!) to and from this turntable. I THINK I remember riding the turntable in the locomotive while it was being turned, but I could be mis-remembering THAT!
EDIT (10Feb2016) -- Yup, I mis-remembered THAT! I recently found found out that this turntable was put here in the early 1990s. It may have been moved from another location. Aerial photographs show that there were TWO turntables around here in 1966. I suspect the turntable I am remembering is the one I labeled "#2" on the pic in the image gallery titled "More Turntables!"
You can walk or bicycle to the cache, but not park anywhere particularly near. The area is very busy on weekdays, but I found easy weekend parking on Main St. to the south. It is NOT necessary to leave the sidewalk to find this cache! UPDATE: Yes, you do have to leave the sidewalk, but just a very few feet along the asphalt. There IS someone living under the bridge.
July 29, 2018 -- The container is now a magnetic thingie.
Bring your own writing implement. And as always, look out for spiders and such beasties!