WASHINGTON, July 15 - Federal Reserve Chairman Alan Greenspan on Tuesday said Congress should be careful before letting a type of bank open branches across state lines. Greenspan said a change that Congress is considering to expand industrial loan corporations could open the door to mixing banking and commerce, which lawmakers have said they want to keep separate. "It is a crucial decision and should not be determined in effect by a relatively small act which is under discussion," the Fed chairman said in response to a question after testifying before the House of Representatives Financial Services Committee.//