Gresley's A4 Pacifics are arguably the most famous locomotives ever built, a status cemented by Mallard's record breaking run on the 3rd July '38. And yet only a year later the glamorous 'streaks' seemed likely to be cast into obscurity by the coming of another world war. So, for only four exhilarating years they were allowed to flourish as their creator had intended and in that time captured the imagination of railwaymen and public alike. With the help of previously unpublished material the author analyses the complex evolution of the A4s - a project that began in 1911 when Gresley was appointed as Locomotive Superintendent of the Great Northern Railway. It is a story with many strands to consider – war, peace and war again, engineering and art, politics and business, recession and social change, the growth of the media and consumerism, the struggle for professional reputations and a growing, deeply damaging international rivalry. All these elements are captured in the story of the A4s in the heady days before conflict ended their brief golden age and Gresley's life came to an end.