Which organization was responsible for nationalizing the railroad system during WWI?

The United Railways Administration was the organization responsible for nationalizing the railroad system during World War I. Established in 1917, it took control of the nation's railroads in order to ensure the efficient transportation of troops and war materials. This move was crucial for the war effort, as it allowed the government to prioritize military logistics and manage the railroads as a unified system rather than as a collection of competing private entities. The administration facilitated coordination between different rail lines, aimed to eliminate inefficiencies, and implemented regulations for safety and schedules that directly supported the war effort.

The other organizations mentioned, such as the War Transportation Board, Federal Railroad Commission, and Railroad Labor Board, although involved in various aspects of transportation and labor relations, did not possess the direct authority to nationalize railroads in the same manner as the United Railways Administration during this critical period. This is what makes the United Railways Administration the correct answer in this context.