I’ve seen a close-up version of this photo of Johnny cash plenty of times (it was a Life magazine cover and album cover as well), but I just found a more complete version, showing the rest of the locomotive, looks like a cold engine somewhere with some fake steam piped in from the left:
Does anyone know where this was taken and which 4-8-4 it is/was?

http://www.ncstl.com/576/576today.htm
That would be the NC&STL 576. Supposedly (from what I've heard), there is still asbestos under the jacketing. I can only imagine what that has done for the boiler. It's just sad...

Boy, don't I feel foolish.
I saw this engine less than a month ago! Why it didn't occur to me which locomotive this could have been, sort of baffles me now, given the location.
I wish I'd gotten a shot of me leaning on the same spot, had I known this at the time.
There's a Johnny Cash museum downtown Nashville, it's odd they had no mention of this as it's a pretty well-known photo of the man, with a local connection.

I recall wondering about that album cover as a kid. Not the location so much as the train makeup. The string of silver painted coaches looked to be a work train? I wonder if that was intentional, or they just happened to be going by?

The caboose appears to be a Southern Pacific bay window type. My first thought was that the scene is on the Colton-Palmdale line, but the LP was issued in 1960, long before that line was even on the drawing board. Might be Soledad Canyon, which would be the closest location to Hollywood. If indeed that's the location, who would have dreamed that in 35 years, commuter trains would be running through here?

Just received a confirmation that the album cover photo was indeed taken in Soledad Canyon, between Lang and Ravenna. There was even a contemporary view of the scene taken from Soledad Cyn. Rd. (for non-Southern Californians, this is northeast of Los Angeles, near the Vasquez Rocks, scene of many movie location shots.)