To combat this, the ARU launched a boycott on Pullman, where ARU would refuse to run trains with the cars. The extensive use of Pullman cars across the country further crippled the railroad industry and stymied rail traffic. The twenty-four Chicago railroads represented by the General Managers' Association rallied together against the boycott. They used strikebreakers to replace striking workers and convinced lines to run Pullman cars with their mail cars. If strikers shut down lines with mail cars, the federal government would have to intervene as it threatened interstate commerce. Indeed, lines were closed and the government found the boycott in violation of the Sherman Antitrust Act.