Union Pacific Railroad Co. (UP) and Missouri Pacific Railroad Co. (MP) and their respective corporate parents filed a joint application with D seeking permission for UP to acquire control of MP. A similar but separate application was jointly filed by UP and the Western Pacific Railroad Co. (WP). These applications were opposed by a number of labor organizations, including respondents Brotherhood of Locomotive Engineers (P) and United Transportation Union (UTU), as well as several competing railroads, including petitioner Missouri-Kansas-Texas Railroad Co. (MKT) and the Denver and Rio Grande Western Railroad Co. (DRGW). MKT and DRGW also filed responsive applications seeking the right to conduct operations using the track of the new consolidated carrier in the event that the control applications were approved. MKT's request specified that it, with its own employees, and at its sole cost and expense, shall operate its engines, cars, and trains on and along Joint Track. DRGW's application indicated that it 'may, at its option, elect to employ its own crews for the movement of its trains, locomotives, and cars to points on or over the Joint Track.' P approved UP's control acquisitions and granted MKT's application for trackage rights over 200 miles of MP and UP track in four States and DRGW's application for rights over 619 miles of MP track between Pueblo and Kansas City. After the control transactions were consummated, P filed with D a 'Petition for Clarification,' contending that D had no jurisdiction to inject itself into labor matters such as crew selection, and asking D to declare that its October 20, 1982, order did not have the intent or effect of authorizing the tenant carriers to use their own crews on routes that they had not previously served. D denied the petition, ruling that its prior decision 'does not require clarification.' P sought 'reconsideration' of D's denial. D again denied the petitions. P petitioned for judicial review of the May 18, 1983, and October 25, 1983, orders. The United States Court of Appeals for the District of Columbia Circuit vacated both orders. It concluded that if D intended to exempt the railroads from the requirements of the RLA, it was required to explain, as it had not done, why that exemption was necessary to effectuate the transactions it approved. D appealed. The Supreme Court granted certiorari.