By Scott DiSavino NEW YORK (Reuters) -Oil futures fell about 3% to a one-week low on Thursday on a tentative agreement that would avert a U.S. rail strike, expectations for weaker global demand and continued U.S. dollar strength ahead of a potentially large interest rate increase. Brent futures fell $2.70, or 2.95%, to $91.40 a barrel by 1:18 p.m. EDT (1718 GMT), while U.S. West Texas Intermediate (WTI) crude fell $2.79, or 3.2%, to $85.69. That puts both benchmarks on track for their lowest closes since Sept. 8. Major U.S. railroads and unions secured a tentative deal after 20 hours of intens…