Díaz created a formidable political machine, first working with regional strongmen and bringing them into his regime, then replacing them with jefes políticos  who were loyal to him. He skillfully managed political conflict and reined in tendencies toward autonomy. He appointed a number of military officers to state governorships, including General Bernardo Reyes, who became governor of the northern state of Nuevo León, but over the years military men were largely replaced by civilians loyal to Díaz. As a military man himself, and one who had intervened directly in politics to seize the presidency in 1876, Díaz was acutely aware that the Federal Army could oppose him. He augmented the rurales, a police force created by Juárez, making them his personal armed force. The rurales were only 2,500 in number, as opposed to the 30,000 in the Federal Army and another 30,000 in the Federal Auxiliaries, Irregulars, and National Guard. Despite their small numbers, the rurales were highly effective in bringing control to the countryside, especially along the 12,000 miles of railway lines. They were a mobile force, often put on trains with their horses to put down rebellions in relatively remote areas of Mexico.
Question: Where did the rurales do most of their "policing" ?
Answer: railway lines