Why did the Pullman Strike disrupt rail traffic across the Midwest?

The Pullman Strike disrupted rail traffic across the Midwest because it evolved from a localized factory walkout into a widespread boycott organized by the American Railway Union (ARU).12

Following the refusal of the Pullman Palace Car Company to arbitrate wage and rent disputes, the ARU initiated a boycott in June 1894, instructing its members to refuse to handle any trains that included Pullman cars. Because many railroad switchmen were members of the ARU, they successfully halted the movement of these cars. When railroads fired these workers in response, other union members walked out in solidarity, causing a cascading effect that paralyzed rail lines. By late June, tens of thousands of workers had quit, effectively snarling rail traffic on nearly all lines west of Chicago. The disruption became a national crisis when strikers interfered with trains carrying U.S. mail, leading the federal government to intervene with an injunction and troops to break the strike.213