United States Steel Corp. announced yesterday that it has agreed to sell Transtar LLC to an affiliate of Fortress Transportation and Infrastructure Investors LLC for $640 million. The transaction is expected to close in the third quarter subject to customary closing conditions, including receipt of certain regulatory approvals, U.S. Steel officials said in a press release. Transtar owns six short lines: the Gary Railway Co. in Indiana; the Lake Terminal Railroad Co., Ohio; Union Railroad Co. LLC, Pennsylvania; Fairfield Southern Co. Inc., Alabama; Delray Connecting Railroad Co., Michigan; and the Texas & Northern Railroad Co., Texas.