Thanks to all for your comments.

I only have one volume of Myrick's RR of AZ series and after digging it out found it covered the "Tucson & Nogales" line. Photos in the book show:

Jan 1946 – 378 w/ 3003 + 3 or 4 cars
date? – 379 w/ 3025 + RPO/bag and coach
Oct 1950 – 378 w/ 2452 + RPO/bag and coach

And it says last run was January 1951. The presence of such a small train and the RPO makes my guess that abandonment was probably prompted due to loss of mail contract as well as few passengers.

Looking at the smaller road steam locomotives assigned to the Tucson Division:

1949 Tucson Div: 1502 1617 1627 1673 1677 1744 1756 1775 1793 1816 1835 1836 2105 2106 2301 2307 2324 2364 2372 2406 2443 3002 3025 (other 5 Atlantics on Western Div.) 3122 3129

1951 Tucson Div: 1617 1627 1645 1673 1677 1722 1751 1788 1799 1800 1824 2105 2364 2406 2408 2419 2443 2452 3025 (only other Atlantic left was 3002 to LA – 3025 now at Traveltown) 3122 3129 (1673 on display at Tucson)

What I was trying to determine is is SP tried a GE 70 ton to replace steam and cut costs like on trains 381/382 Gila Tomahawk. If they tried one, it would have been between Feb 1950 and Jan 1951 which would have to be 5110-5113. I'm not sure they bothered and if so it must of been for just a few test runs. Available info suggests that there were more assignments of SP's GE 70 tons in Arizona besides the 5115 on the mixed trains out of Benson until 1962 (Rio Grande Div.) and 5112 or 5118 on No 381/382 until it ended by 1954.