This paper examines the usefulness of the category ‘interest’ in the history of technology through a case study of the failed attempt to introduce atmospheric railway traction in Britain in the 1840s. It argues for a reflective deployment of the term in question, sensitive to contemporary usages. A schematic typology of railway ‘interests’ shows just how various these were. Financial interests, for example, were not the same as professional ones; or an investor might feel the pull of contrary interests, depending on whether she was checking her dividends or taking a train. The history of atmospheric traction demonstrates the interlocking of railway ‘interests’, and their utility in explaining the success or failure of railway technologies. As might be expected, determinism, whether technological or ‘social’, does not answer the case. ‘Interest’ relations are contingent, and so must be understood the failure of the atmospheric railway.