U.S.-based railroad Union Pacific's credit measures have improved year-over-year for the 12 months ended March 31, 2012. We are raising our long-term ratings, including the corporate credit rating, on Union Pacific to 'A-' from 'BBB+'. The outlook is stable, reflecting our expectation for pricing to remain favorable and operating efficiency to improve, resulting in further earnings growth, increasing cash flow generation, and stable credit metrics. NEW YORK (Standard&Poor's) April 23, 2012--Standard&Poor's Ratings Services said today that it raised its long-term ratings on Omaha, Neb.-based Union Pacific Corp. and subsidiaries, including the corporate credit rating to 'A-' from 'BBB+'. "The upgrade reflects Union Pacific's improved earnings, stronger operating efficiency, and strengthening credit measures," said Standard&Poor's