No. 1 U.S. railroad Union Pacific Corp reported a jump in quarterly profit Thursday that beat analyst estimates as a growing economy lifted freight volumes and higher freight rates boosted revenue. Shares of the company were up 4 percent in light premarket trading. Still, the company sounded a note of caution on the outlook for the U.S. economy amid uncertainties following a significant drop in oil prices over the past several months. “Clearly, one of the biggest uncertainties is the outlook for energy markets, which will bring both challenges and opportunities as we move ahead,” Chief Executive Jack Koraleski said in a statement. The Omaha, Nebraska-based company earned a fourth-quarter net profit of $1.43 billion, up 22 percent from $1.17 billion a year earlier. It earned $1.61 a share, up 27 percent from the fourth quarter of 2013. Analysts had expected earnings per share of $1.51. Revenue in the quarter was $6.15 billion, up 9 percent from a year earlier and slightly above analyst expectations of $6.1 billion. The railroad reported higher revenue in almost all its freight services, with the exception of automotive, which was unchanged. Revenue from agricultural products was up 9 percent, while industrial products and intermodal, or consumer goods, was up 15 percent and 11 percent, respectively. The drop in oil prices meant that in the fourth quarter Union Pacific paid an average of $2.97 for a gallon of diesel, a 6 percent decrease from the $3.15 it paid in the fourth quarter of 2013. Despite a 6 percent increase in carloads, the amount Union Pacific spent on fuel fell more than 10 percent to $813 million from $905 million.