Ironically, the case had nothing at all to do with whether or not corporations were persons. Furthermore, headnotes do not carry the weight of the law and are not part of the decision.
Yet, as mentioned, court cases that followed Santa Clara County v. The Southern Pacific Railroad have been decided upon as if the 1886 case legally gave personhood to corporations.
There are many court cases which have protected corporations as though they were natural human beings. The following are three examples.
The first example is First National Bank of Boston v Bellotti, 1978. In this case, the court found that political donations by corporations are protected by the first amendment of the Constitution, which guarantees "people" free speech. It claimed, in essence, that money is speech. Obviously, some "people" are able to speak louder than others.
Two court cases protected corporations based upon their "rights" as stated in the fourth amendment to the Constitution.
In 1906, the court case Hale v Henkel decided in favor of corporations not having to present documentation showing whether they are or are not violating US Antitrust Laws.
In the 1978 case Marshall v Barlow's Inc. it was decided that a surprise inspection by the Occupational Safety and Health Administration violated the fourth amendment rights of corporations. It was decided that surprise inspections were "unreasonable searches". Not only did the decision marginalize worker safety by denying OSHA the right to inspect a workplace for safe working conditions, but a chemical plant cannot be inspected without warning to ensure that it's not blatantly polluting. Do you live anywhere near a chemical plant? Think about it.
With the stroke of a pen, corporations didn't gain personhood, yet began to be treated by US courts as if they had.
What Opportunity did Corporations use to Pursue Personhood?
Following the Civil War, people of African descent were no longer allowed to be owned by Whites in any part of the US. Congress decided that, to solidify this law, an amendment to the Constitution was needed. That amendment was the fourteenth amendment and was passed and ratified in 1868.
Section 1. of Amendment XIV reads as follows: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
Freedom would do those of African descent no good if they were not protected under the laws of The United States as full-fledged citizens. The fourteenth amendment was ratified to ensure that everyone in every state in the US realized that people of color were citizens of The United States of America and possessed all of the rights identified in the Constitution of The United States.