Page "Labor history of the United States" Paragraph 18
The railroads were able to get Edwin Walker, general counsel for the Chicago, Milwaukee and St. Paul Railway, appointed as a special federal attorney with responsibility for dealing with the strike.
Walker went to federal court and obtained an injunction barring union leaders from supporting the boycott in any way.
The court injunction was based on the Sherman Anti-Trust Act which prohibited " Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States ".
Debs and other leaders of the ARU ignored the injunction, and federal troops were called into action.