I thought that it looked a lot like a Morenci Southern engine. I have been comparing it to a couple of photos that I have. Like you say the air compressor isn't there. The stack, location/style of the domes and bell and the cab sure look right.
I was thinking that it might be the location shown in the aerial below. This is down towards the southern end of the line (maybe the bridge that you mentioned). I haven't had a chance to make it over to the two major bridges on the southern end yet. I can say though it is definitely not the Owl Canyon bridge.
This is making me want to head over there for another grade hike. This bridge isn't too far from a dirt road/cow path as shown in the aerial.
Dave