Mediation means that both parties reach an agreement on all points. Arbritration means a third party sets the terms not the two parties. These RRs have too much at stake and will not agree to any compromise. Arbritration will then be offered which the RRs and probably Amtrak will not accept. The RRs will just keep asking for mediation extensions until the STB finally says no more.
IMO STB will set conditions which these RRs will take it to court. Shopping for a friendly judge probably in the 5th circuit court of apppeals jurisdiction. The 5th circuit has many RR friendly judges and the 5th circuit appeals court is too business friendy.