The book The Modoc by Bowden and Dill, has 2 references that I was able to locate in a short glance, referring to the AC - 9's as having " ... clearance problems on Donner Pass " (pg. 127 ), and " Another notice ( stenciled )on the side of the cylinders ( and also the tender ) informs the observer that these engines had excessive width, a fact that prevented them from coming to Sparks by way of Donner Pass, necessitating the circuitous route by way of Dunsmuir and Klamath Falls " ( photo caption pg. 219 ).
The AC - 9's also weighed 20,000 lbs more than any other AC's, and they were in fact Espee's heaviest steam locomotives. The claim of normal per - axle loadings is misleading, as the drivers will carry more weight than lead or trailing truck axles, or even tender axles on most classes.
One more point to ponder : If the AC-9 Gila Monsters did indeed fit the clearance standards for Donner Pass, why did Espee run them from Roseville to Sparks via Dunsmuir & K - Falls ???