The first three articles of the U.S. Constitution "<span>explain the powers delegated to the national government," since they establish the three main branches of the federal government. </span>
Immigration does not fall under the domestic policy. A domestic policy differs from foreign policy, which refers to the ways a government advances its interests in world politics.
They did it so as to transport commodities from the continent to the seaports.
The two regions had dreadful roads and it was extremely difficult to transport commodities to seaports which would then be taken to factories worldwide. They needed railroads to move those commodities since they usually came in high amounts so they built railroads all across the continent out of their own need.