Chapter XXXI: Section 9: , of Article 1, of the Federal Constitution says: “No

preference shall be given by any regulation of commerce in revenue to ports of one State over those of another.”

Congress itself cannot establish any differential that would give one port of the United States an advantage over another port. But what the Constitution forbids Congress to do the railroads can do and have done, by manipulating the rates on exports and imports, thereby making business flow to whatever ports they please.