I'm not sure if people get what the NMB does. The mediator is there to facilitate negotiations when their presence is requested by either party in the negotiation. They can not force one side or the other to change their position on ANYTHING in the negotiation process. They are there to facilitate only. They can require the parties to meet on certain dates. They can require the parties to be prepared to discuss specific contract sections/items, but they can not force either party to a position. The only time the NMB has any binding influence in the process is in their responsibility to declare an impasse in negotiations, thus starting the 30 day cooling off period. If you are making any progress in direct negotiations, you are typically better off not filing for mediation. If you think getting a contract is taking too long as it is, just invite the Federal Government to join the process and see what slow progress really means.

p.s. Even when the NMB expends all of their resources and finally recommend releasing the parties to self help (aka strike or lockout), the Presidential Emergency Board can still prevent the self help from taking place.