The decision came after four days of hearings triggered by the recently proposed merger of the Burlington Northern Santa Fe (BNSF) and the Canadian National railways.
STB noted that the railroad industry has consolidated aggressively in recent years, leaving only six large railroads remaining in the U.S. and Canada.
"But merger implementation has not typically gone smoothly, and indeed the railroad industry and the shipping public have not yet fully recovered from the service disruptions associated with the previous round of mergers," the agency said.
STB also said that testimony at the hearing confirmed that a BNSF/CN combination would likely touch off responsive mergers among the four other large railroads. Therefore it has decided not to permit further major rail mergers until the regulations are revised.