According to Ohio State University's eHistory website, the Northern Securities case refers to the prosecution of the Northern Securities Company by Theodore Roosevelt's administration for violations to the Sherman Act. The case reached the United States Supreme Court in 1903 and the court found in favor of the Roosevelt administration's anti-trust efforts against the Northern Securities Company, leading to the dissolution of the company, notes eHistory.
In the syllabus of the Northern Securities Company v. United States Supreme Court case, as published on the Cornell University Law School website, two companies are identified as having colluded to form the Northern Securities Company. These two entities were the Great Northern and Northern Pacific Railway. The syllabus explains that the purpose of their collusion was to form a holding company that would control the shares of both constituent companies. In so doing, Northern Securities was able to control key logistical rail lines with the intent of eliminating competition. The case was decided in favor of the United States on November 3, 1903.
Laws.com states that this case was the first relevant application of the anti-trust Sherman Act. Following the Northern Securities case, it was used as precedent for other anti-monopoly cases that were brought before the U.S. Supreme Court.