Fallout: Railroad- you can serve the institute but only after you talk to Desmonda and also you can serve the brotherhood till you decide to speak to the insider to begin the battle , you can do Minutemen for all endings Brotherhood- you can work with all till you decide to go and do the mass fusion, once the mass is done you are enemies with the railroad when you kill them and the institute right after you begin mass fusion, you can work for Minutemen. Minutemen- you can go work all till the defend the castle quest comes then you can no longer go to the institute, other factions are friendly even after the ending. Institute- when you work for the institute without getting the insider from the railroad then railroad will tell you that they heard disturbing things about you working in the institute and will be kicked out when you reach mass fusion quest. Begin it and brotherhood will be enemies with you if activated in institute, not sure if this ending is possible to work with Minutemen.