Oil companies and railroads have offered U.S. regulators a proposal for a tougher tank-car design and a plan to phase out older model rail cars, according to two people involved in the talks. The American Petroleum Institute , which represents companies including Exxon Mobil Corp., and the Association of American Railroads, whose members include Berkshire Hathaway Inc.’s BNSF railway, agreed on a proposal for tank cars that have steel shells a half-inch thick, slightly thicker than current standards. The two groups also agreed on a goal for a three-year phase out of older models known as DOT-111s that safety investigators have said are vulnerable to rupture in a crash, said the two sources familiar with the issue. To contact the reporter on this story: Jim Snyder in Washington at [email protected] To contact the editor responsible for this story: Steve Geimann at [email protected]