1958 San Francisco

found on https://www.facebook.com/pages/Neat-Old-Stuff-and-Yesteryear/154872514719231

Steve let me know that this image showed up on Shorpy 3 years ago, and there are a lot of notes and information gleaned from it
http://www.shorpy.com/node/13324

like this observation about the pusher COE behind it:

The red Dodge truck following the cable car is almost certainly a repair vehicle sent out by the Municipal Railway (MUNI). Check out its oversize pushing bumper and tool boxes at rear. It appears to be assisting cable car 502, which has undergone some type of mechanical mishap and is now out of service. Witness the "Take Next Car" sign in the front window..

The cable car is headed for the Powell Street turntable, and if it can't be fixed there the Dodge will most likely have to push it all the way back to the car barn at Washington and Mason Streets. It will be a long push -- mostly uphill.

Cable cars were (and still are) notoriously finicky and tend to go flooey at inconvenient times. This car could have 'lost its grip,' meaning the mechanical device that grips onto the moving subsurface cable that pulls the car along. Alternately, it might have suffered some sort of brake malfunction. Whatever the case, MUNI didn't want any riders on this trip.