In the case of a railroad worker accident brought to court in the 1900s,one of the defenses the railroad company used against the plaintiff was the “fellow-servant” doctrine.What does this refer to?

The claims for such incidents are usually settled out-of-court, as between fellows. The company is not a separate entity above the employee, but a colleague and a caretaker. The injured employee had assumed responsibility of the risks involved. The accident is attributed to the negligence of the co-worker.