GP9 roof details

[Sumpter250]

I thought about radio control - but why two boxes?

Those may be exhaust scrubbers used to clean up the exhaust

But why two boxes, if they are exhaust scrubbers, then why isn't one of them mounted over each of the exhaust stacks, instead of on either side of one exhaust stack, and closer to the cab.
In the original photo, the second unit (and it's hard to really see) seems to show that the boxes are identical, and having distinct ends each, and mounted, each facing opposite the other.
I couldn't quite make out the name of the mine,so I took the photo into Corel photo, and was able to read Morenci Mine. This is an open pit copper mine. I did find these pics,http://www.westcoastrailforums.com/view_...p_to=10328 but so far no information about the "boxes", but contacting them would probably produce an answer to this question.