Okey doke...now that I think about it, I saw a buffalo or two in some semi-mountainous portions of Wyoming, from the UP mainline. Other than that the only buffalo I remember seeing were the little preserve at the particle accelerator facility outside of Chicago, so I'm no authority on buffalo. We don't have 'em out here in Californy.
Although I do note that in three out of the four photos shown, the buffalo are actually in a nice flat area NEXT to the mountains, rather than in 'em--not quite the same thing.
Some other western-type things about buffalo and trains: There were "buffalo hunts" sponsored by railroad lines--not really hunts as such, but trains carrying men with rifles, who rode through areas where buffalo herds were found, shooting buffalo from the train. Typically they didn't stop to make any use of the buffalo they shot, and it wasn't exactly sporting, but I imagine it was great fun.
"Old West" towns are also appropriate in mountainous regions--in some of the more remote areas of California's northcoast and the Sierras, I see towns that don't look too far removed from the Old West other than a few pickup trucks in place of horses, and satellite TV antennas.
It seems like there would be some good possibilities for operation on your layout, Cookie: the mountain logging operation supplies lumber to the prairie-based boomtown, which in turn supplies foodstuffs to the loggers.
Hmmm...didn't Buffalo Bill get his name supplying buffalo meat for some similar enterprise?