Its a difficult situation. You are seeking employment from RRB through a centralised recruiting agency which very clearly mentioned that the selection is only provisional and actual appointment is based on the requirement of the Bank concerned. Yes, as informed by you, there is injustice meted out to you all. As suggested by learned member, you may approach High Court concerned (under whose jurisdiction the RRB Head Office is situated) for relief and/or information.
Normally any RRB shall be sponsored by a bigger bank (usually a Public Sector Bank). Find out which bank sponsored this RRB and write letters to that Bank too. When you decide to file a case, see that the sponsoring bank too is included.