I'm not sure of the elevation at Otowi, but I always thought the "low point" on the Narrow Gauge (in more ways than one to some folks!!) was Farmington. At any rate the Santa Fe Branch was long gone by the 1950's so, it really doesn't fit into the plan.
There is a shot in one of the NG Pictorials of a USA diseasal dumping ballast on a frt north of Farmington. I think they determined using it to switch in Durango and go to Farmington was about all it could deal with.
One other major downfall of narrow gauge that is not a problem "Down Under": Eventually the narrow gauge ends and everything has to be transferred to or from broad gauge. Not a problem if all you have is 3' or 3'6"!