This book is a comprehensive history of all twenty-six classes of four coupled tank engines commissioned by the Great Western Railway or built at their Wolverhampton and Swindon Works, from the Broad Gauge 2-4-0 and 4-4-0 tanks of the 1840s and 1850s to the well known Collett 0-4-2 branch line engines of classes 48XX (later renumbered 14XX) and 58XX of the 1930s. As well as the Broad Gauge engines, the strange looking Covertibles' of William Dean, a number of experimental one-off' designs, the numerous Wolverhampton 0-4-2Ts of the 517' class and the Swindon built 2-4-0 Metro Tanks' are described with - where known - their allocation and operation. The book includes twenty weight diagrams and nearly 300 photographs, over 50 in colour. The four-coupled tank engines absorbed by the Great Western from other companies at or before 1923 will be featured in a separate volume to follow.