The Railway Labor Act Is a Statute, a System of Laws That Governs Labor Relations on the Railroads

Labor relations on the railroads are not governed by the National Labor Relations Act like most jobs are. They are in fact governed by the railway labor act, a system of laws that was essentially put in place in the 1920s after decades of seismic railroads strikes. If any one of them in a situation like this rejects the tentative agreement and decides to go on strike, that couldn't affect trigger a national rail shutdown because the other unions have their contracts to not cross the picket line.