By Medha Singh and Bansari Mayur Kamdar Sept 15 (Reuters) - Shares of Union Pacific Corp UNP.N and Norfolk Southern NSC.N rose on Thursday after major U.S. railroads clinched a tentative deal with unions for better pay and working conditions, narrowly averting a rail shutdown. The deal also offers some relief to investors already grappling with the impact of the Federal Reserve's relentless interest rate hikes to curb surging inflation. Wall Street suffered its worst day in more than two years on Tuesday after a hot August inflation report fueled bets of more aggressive monetary policy tightening.