The L-131's didn't have a very long rigid wheel base, but they did have considerable overhang on the curves. Before they sent one over the hill, they made a wood mockup of a 3600 and attached to the front of a smaller 3500 class 2-8-8-2 to see if they would fit the cuts and curves. Eldon Morgan fired the 3500 on that trip. It cleared everything just fine, the 3600's began to run out their last miles out of Alamosa.
The F-81 class 2-10-2's had lateral motion devices on the front and rear sets of drivers, that gave them enough "give" to squeak around the 16 degree curves. They also had a section gang up at Fir to check the track daily and make sure none of the curves got wide gauge.