One of the early drivers of historical GIS was the development of national historical GISs. These systems usually hold all of a country's census and related statistics from the nineteenth and twentieth centuries. As such they have represent an extremely valuable resource, but at the same time they were and remain extremely expensive and time consuming to build. Was the investment worthwhile? This paper takes one of these systems, the Great Britain Historical GIS, and explores how it was built, what methodologies were developed to exploit the data that it contains, and provides an example to demonstrate how it made possible a unique analysis of railroads in Wales before the First World War.