Yup, pretty much anything it wants!  

If no agreement is reached at the end of the cooling off period, and absent Congressional action, the parties may engage in self-help; including, strikes "CHAOS™", lockouts, and company imposed unilateral changes in terms and conditions of employment.

https://www.contract2021.org/understanding-the-railway-labor-act/>

It can make any changes it wishes, but will not want to destroy its chance of surviving the long term health of the company.   whatever it imposes, it is the new CBA (ie, the green book), that is now binding on itself.   So, they will generally add changes in the language that make it easier to implememt future unilateral changes if needed.   PHI did that.   

In the end, it amounts to anything!