Adds details, background April 21 (Reuters) - U.S. railroad operator Union Pacific Corp UNP.N posted a 21.5% jump in first-quarter profit on Thursday, benefiting from higher shipments due to a rise in U.S. factory production and price hikes. Price increases amid strong demand for high-margin automotive and intermodal shipments have helped U.S. railroads cushion the impact of persistent supply chain disruptions, labor shortages and inflation. Operating ratio, a key profitability metric, improved to 59.4% from 60.1% a year earlier.