This thesis describes the rail gauge decision-making processes of the Australian colonies in the period 1850 - 1901. Federation in 1901 delivered a national system of railways to Australia but not a national railway system. Thus the so-called 'standard' gauge of 4ft. 8½in. had not become the standard in Australia at Federation in 1901, and has still not. It was found that previous studies did not examine cause and effect in the making of rail gauge choices. This study has done so, and found that rail gauge choice decisions in the period 1850 to 1901 were not merely one-off events. Rather, those choices were part of a search over fifty years by government representatives seeking colonial identity/autonomy and/or platforms for election/re-election. Consistent with this interpretation of the history of rail gauge choice in the Australian colonies, no case was found where rail gauge choice was a function of the disciplined search for the best value-for-money option. To the extent to which economic variables were considered in rail gauge choice, the initial cost of construction became the proxy for value in building and operating railways. In Australia engineers' opinions in the role of advice to governments or parliamentary committees were substituted for rigorous evaluation. These and other findings were reached by reference to primary documentation including companies' minute books, official correspondence and reports of the proceedings of parliamentary committees, generally missing from earlier studies. This study analyses and interprets them by reference to the history of the 'standard' gauge, to the economic and constitutional settings of the time, and by reference to rail gauge choices and decision-making processes in other large developing economies of the time, notably Brazil, India and the United States. In so doing a new narrative and interpretive study of early rail gauge choice in Australia has been created. The finding that previous studies of the early development of Australian railways either ignored or misread important documents has led to a reappraisal of the part played by several individuals, including engineers in all four colonies, Lieutenant-Governor LaTrobe and WE Gladstone in his role as Secretary of State for the Colonies and for War.