The correct answer is A) the Northeast.
The region of the country that has the greatest concentration of railroad lines is the Northeast.
Since the Industrial Revolution times, railroads played an important role for the transportation of products and people in the Northeast of the United States. That part of the country was the most populated in that time, so the movement of people and products was somehow impressive on a daily bases.
The movement of people even now, from and to Boston, Chicago, Philadelphia, Maryland, and New York is more than considerable. Millions of people use the train or Amtrak to commute.
