By Trevor Hunnicutt and Steve Holland WASHINGTON (Reuters) -Major U.S. railroads and unions secured a tentative deal on Thursday after 20 hours of intense talks brokered by President Joe Biden’s administration to avert a rail shutdown that could have hit food and fuel supplies across the country and beyond. Biden called the deal a “big win for America” and for tens of thousands of rail workers. Thanking business and labor, the Democratic president promised more worker-company agreements in the future. “I’m optimistic that we can do this in other fields as well,” Biden said. “Unions and managem…