Complex sentence: During his tenure as Postmaster General, under the new Federal Constitution, the Post Office establishment was reorganized and, the old horse and mail was first carried in stagecoaches on main routes, displacing the old horse and rider system.
Split into simpler sentences: During his tenure as Postmaster General, under the new Federal Constitution, he reorganized the Post Office. They began to transport mail by stagecoaches on main routes, displacing the old horse and rider system.