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That must run through Glacier Park which I grant you has hills. However I drove east to west across Montana once and the rest of the state is flatter than a pancake.
In August of 1974 I was riding my bicycle down highway US-2 and crossed into Montana from Idaho, and then six days later from Montana to North Dakota; yeah, it is a fair bit flat east of East Bank, Montana, but west of there the only flat surfaces are in Flat Head County.