Feb 28 - Standard & Poor's Ratings Services said today that it has assigned its 'BBB+' rating to Burlington Northern Santa Fe LLC's (BNSF's) $625 million senior unsecured debentures due 2022 and $625 million senior unsecured debentures due 2042. BNSF will use proceeds from the debt issuance for general corporate purposes including working capital, capital expenditures, the repayment of outstanding debt, and distributions. The ratings on BNSF reflect the company's strong competitive position