3' gauge was associated with some pretty good grades. The Keeler-Laws branch until 1938 reached into Nevada via Montgomery Pass IIRC. The NdeM 3 foot gauge had grades too, for sure on the Acambaro line and the Vera Cruz and Oaxaca runs as well. Gerald Best's book has some nice shots of the beautiful Alco 2-6-6-2's on the hills both to Vera Cruz and Acambaro. Once again IIRC the line to Oaxaca had close to a canyon horseshoe curve that experienced a number of wrecks.

There was a reason for needing the power of those 2-6-6-2's, which were inside frame but still too heavy for what remained of the narrow gauge system after the postwar loss of the longer routes to standard gauge. Some of the last narrow gauge outside frame 2-8-0's were regauged in the NdeM shops.