Representatives from the coal industry testified in Washington Wednesday about rail service issues that could put a strain on the nation’s power grid.Coal use saw a resurgence last year, as higher natural gas prices and a stronger economy created more demand for electricity generated from coal.But coal producers and consumers struggled to take advantage of those favorable conditions because of poor rail service, industry leaders told the Surface Transportation Board.Coal shipments have been delayed because of lack of train crews, lack of locomotives and cars, and because of mechanical problems, they said.