RBA,

The following link from ALPA gives a pretty good one page summation of the process and the steps along the way starting with Section 6 notification all the way to Self Help. I realize that your question asks more than simply the steps along the 'Contract road', but I'll let someone else chime in on whether the NMB has simply begun parking airlines in mediation because they're "too big to strike"

crewroom.alpa.org/pclmec > Railway Labor Act