Montana's busiest airport is located in Billings, in the south-central part of the state. Missoula, Helena (the state capital), Great Falls, and Bozeman have large airports, while a number of other cities are serviced by small regional airlines like Great Lakes, Sky West, and Frontier. Still, most visitors will arrive, and get around, by car. Montana is infinitely drivable, and an extended road trip is one of the best ways to see the state. Three major interstates run through the state: I-94 links North Dakota with Billings. I-90 runs from Sheridan, Wyoming, to Billings, on through Bozeman and Missoula, and then into Idaho. I-15 links the Canadian province of Alberta with Western Montana. Intercity bus links are provided by Greyhound, Powder River Trailways, and Rimrock Trailways, while the Airport Shuttle Express of Calgary, Canada, provides charter bus services from Calgary, Lake Louise, and Banff to numerous Montana towns. Train service is available via Amtrak's spectacular Empire Builder, the 2,200-mile stretch of track between Chicago and Seattle. Trains stop a dozen times in Montana, connecting Libby, Whitefish, West Glacier, Essex, East Glacier, Browning, Cut Bank, Shelby, Havre, Malta, Glasgow, and Wolf Point.