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.