Historical GIS is now a well established part of the discipline of history. One of the early drivers of historical GIS was the development, in a number of countries, of national historical GISs. These are systems that usually hold all of a country’s census and related statistics through the nineteenth and twentieth centuries. As such they have the potential to represent an extremely valuable resource, but at the same time they were and remain extremely expensive and time consuming to build. The poses the question, therefore of whether such investment was justified. This paper takes one of these systems, the Great Britain Historical GIS, and explores how it was built, what methodologies then needed to be developed to exploit the data that it contains properly, and finally gives an example of one analysis that it allows that would simply not be possible without this large-scale investment of resources. The example focuses on the impact of the development of the railways in Wales before the First World War.