WASHINGTON, Sept 16 (Reuters) - U.S. Agriculture Secretary Thomas Vilsack met with Warren Buffett last week to urge the billionaire investor to make sure his BNSF railroad is ready for an expected record corn and soy harvest this year.

Vilsack said on Tuesday that Buffett, who heads the sprawling conglomerate Berkshire Hathaway, recognized the challenge and indicated his company was taking steps.

"I said, 'Warren, you've got to make sure that your railroad understands what's going on here,'" Vilsack said he told Buffett during a 45-minute conversation. "There is pressure now, but as soon as this crop is harvested, there will be more pressure."

Speaking at a conference sponsored by Growth Energy, a biofuel trade group, Vilsack said BNSF was making "significant" investments. "It's a long-term issue."

Backlogs along U.S. rail lines became a major concern for a number of commodities markets this year.

In June, for example, U.S. officials ordered BNSF and Canadian Pacific Railway Co to report their plans to clear a backlog of grain cars after months of service delays blamed on harsh winter weather and high freight demand.

BNSF is a unit of Berkshire Hathaway. The railroad, which was struggling from the Great Recession when Berkshire bought it for $26 billion in 2010, returned a $3.8 billion profit last year.

One major source of profit, oil by rail, has become controversial, with some commodities producers saying railroads, including BNSF, are prioritizing the shipments of crude at the expense of other cargo. This has been denied by BNSF.

Berkshire Hathaway did not immediately reply to a request for comment on Tuesday. Canadian Pacific also did not immediately reply to requests for comment.

Vilsack on Tuesday praised BNSF for its work to improve service for farmers this year.

"If we had made as much progress with Canadian Pacific, we'd be in a little bit better place than we are today," he said.

(Reporting by Ayesha Rascoe; Writing by Luciana Lopez in New York; Editing by Cynthia Osterman)