How did railways give france a strategic advantage wwi?

They really didn't, the French had no advantages over Germany when looking 1v1 up to 1917 and America's entrance. The only reason Germany didn't completely destroy France is that they were fighting a two front war until shortly before America's entrance and then the Grant strategy of the US Civil War was used. The Allies had more men and equipment then the central Powers so they just out spent men and material.