August 6th, Twain's party departs Missoula on the Northern Pacific railway. This particular train had two special cars attached carrying the newly appointed receiver for the bankrupt railroad and the Supreme Court judge who had appointed him. Twain did not join them. They traveled through the Bitterroot Valley, ancestral home of the Salish people. They passed the site of the "starvation winter" of 1883-1884, and on through the Flathead Indian Reservation. Although they did not realize the significance of the area, the route traversed where the massive ice dam of Lake Missoula was, on the Clark Fork River 15 to 17,000 years previous. The dam was a lobe of the Cordilleran Ice Sheet that came down Purcell Valley. The lake is believed to have covered 500 square miles and the dam possibly 2000 feet high. Twain's party arrived in Spokane that night, at 11:30, and put up at the Spokane House. He gave a lecture on the 7th and spent the 8th touring the economic disaster zone that Spokane had become. Clara treated the hotel guests to a Chopin nocturne and they departed that night at 11:30. They boarded the Great Northern railroad again and headed west. Though they were unaware of the significance of the region they traveled through the Channel Scablands, the results of the massive and repeated floods from Lake Missoula. The morning of the 9th found them in Leavenworth, Washington. The train, with an additional engine, drove up through Tumwater Canyon to a point where the eastern portal of the first Cascade tunnel would be located. An additional engine was added to the rear of the train and Twain was allowed to ride with the engineer along the switch backs over Stevens Pass and down to the site of Wellington. Wellington was the western portal of the original Cascade tunnel and the site of one of the most disastrous avalanches in railroad history. The train continued down the slope to Martin Creek and through the horseshoe tunnel. then down along the Tye Creek, through Skykomish and on to Tacoma, Washington. 

 https://twainsgeography.com/content/section-5-rockies-and-over-cascades