The Hi-Line in Montana is roughly the Northern-most region that stretches from the Eastern front of the Rockies to the North Dakota. The original Great Northern Railroad originally cut through and defined it, but now the path is usually taken to be US Highway 2.
I grew up near there.
Nancy pointed to a National Geographic piece on the place. I suspect even most Montanans haven't seen much of it - perhaps only the piece immediately to the East of Glacier Park. But if you want to explore prairie, this is an excellent destination and an exotic place that happens to be within the US. Montana Magazine has frequent articles on the region for those who might like a bit more information.