Standard & Poor’s warned Wednesday that Warren Buffett’s bid for BNSF Corp. could sap the liquidity and capital position of the legendary investor’s insurance operations at Berkshire Hathaway Inc., jeopardizing the company’s AAA rating. The ratings agency placed its ratings on Berkshire Hathaway on CreditWatch with negative implications after Berkshire’s announcement Tuesday that it will buy the railway for $26.3 billion. Standard & Poor’s said it expects the transaction will reduce liquidity and "capital adequacy” of insurance operations.