Tracks of the Northern Pacific Railroad (NPR) reached Montana from the west in 1881 and from the east in 1882. However, the railroad played a major role in sparking tensions with Native American tribes in the 1870s. Jay Cooke, the NPR president launched major surveys into the Yellowstone valley in 1871, 1872 and 1873 which were challenged forcefully by the Sioux under chief Sitting Bull. These clashes, in part, contributed to the Panic of 1873 which delayed construction of the railroad into Montana. Surveys in 1874, 1875 and 1876 helped spark the Great Sioux War of 1876. The transcontinental NPR was completed on September 8, 1883, at Gold Creek.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What years were the railroad challenged by Chief Sitting Bull?
1871, 1872 and 1873