Here's a reason why that court case was a landmark.
In 1886, . . . in the case of Santa Clara County v. Southern Pacific Railroad Company, the U.S. Supreme Court decided that a private corporation is a person and entitled to the legal rights and protections the Constitutions affords to any person. Because the Constitution makes no mention of corporations, it is a fairly clear case of the Court's taking it upon itself to rewrite the Constitution.
Far more remarkable, however, is that the doctrine of corporate personhood, which subsequently became a cornerstone of corporate law, was introduced into this 1886 decision without argument. According to the official case record, Supreme Court Justice Morrison Remick Waite simply pronounced before the beginning of arguement in the case of Santa Clara County v. Southern Pacific Railroad Company that:
The court does not wish to hear argument on the question whether the provision in the Fourteenth Amendment to the Constitution, which forbids a State to deny to any person within its jurisdiction the equal protection of the laws, applies to these corporations. We are all of opinion that it does.
The court reporter duly entered into the summary record of the Court's findings that: The defendant Corporations are persons within the intent of the clause in section 1 of the Fourteen Amendment to the Constitution of the United States, which forbids a State to deny to any person within its jurisdiction the equal protection of the laws.