What disagreement led to legal conflicts between the mexican and european-american landowners in the 1880s?

The transcontinental railroad was completed in 1869, and railroad companies wanted to expand into land that was tribal land. Settlers also want to claim the land on the frontier. This causes conflict, because American settlers were trying to claim land that, according to Mexican law, belonged to the community of the natives whose families lived there. However according to American law, ownership is through purchasing a deed. This difference of law caused conflict.