Conversation so far:
Who began The Great Sioux War of 1876?
By the early 19th century, the Northern Cheyenne became the first to wage tribal-level warfare.
Who was Sitting Bull?
Sitting Bull in Standard Lakota orthography, also nicknamed Húŋkešni  or Slow) was a Hunkpapa Lakota leader who led his people during years of resistance to United States government policies.
What did Sitting Bull's band of Hunkpapa do preceding The Great Sioux War of 1876?
Sitting Bull's band of Hunkpapa continued to attack migrating parties and forts in the late 1860s.
When was Sitting Bull's attacks on the Northern Pacific Railway?
When in 1871 the Northern Pacific Railway conducted a survey for a route across the northern plains directly through Hunkpapa lands, it encountered stiff Lakota resistance.
Who was Sitting Bull attacking in 1871?
Sitting Bull and the Hunkpapa attacked the survey party, which was forced to turn back.

Question: What year was this?
Self-contained rewrite: What year was Sitting Bull's attacks on the Northern Pacific Railway?