English: Areas in white have a population density of under 2 per square mile – and thus represent frontier territory^[1]. Unlike in the previous census in 1900, the west coast and the east are now connected by inhabited corridors. The frontier line has disappeared. The northern-most corridor follows the path of the Northern Pacific Railway. A second corridor just north of the Great Salt Lake follows mostly the Oregon Trail. To the south, a third, not quite "completed" corridor follows parts of the Atlantic and Pacific Railroad.