Sten De Geer's classic monograph-length geographical interpretation of the American Manufacturing Belt (AMB) published in 1927 concludes with a major section on the importance of 'railroad lines and systems'. This chapter examines the operational issues involved in the construction of a general-purpose historical geographical information system (GIS) of the US railroad network, by means of a study of the part of that network within the AMB. It outlines briefly how a specific study might be undertaken that utilizes both the general-purpose GIS and specific accounting data derived from annual reports. The railroad historical GIS forms part of a wider, long-term project to investigate the industrial development of the north-east USA in the second half of the nineteenth century. Since the GIS was always intended to be a multi-purpose resource, eventually to be made widely available to a variety of academic users, it needed to support integration with a range of different datasets, e.g. from the mining or metal manufacturing sectors.