The significant effects of competition, franchising and vertical separation of rail infrastructure from train operation on the level of transaction costs are often anecdotally described in the literature. Although it has been shown that franchising has an effect on total costs, there is very little empirical evidence on whether franchising has an impact on the level of transaction costs over time. One reason for this is, of course, the limited systematic work on the measurement of transaction costs in railways. This paper builds on recent work that applied a top-down approach to transaction cost measurement to identify the size of the transaction sector within rail firms in different EU countries. In cross-country comparison particularly, British train operators turned out to be associated with high levels of transaction costs. However, since the previous work focused on a single fiscal year it did not show any longitudinal effects within one institutional environment or country. Therefore, this paper focuses on British franchised passenger train operating firms and aims to reveal how the transaction sector within those firms has changed over the period 1996/1997–2007/2008 and whether the franchise contract details, such as contract length or the franchising regime, matter. It also aims to estimate the resulting changes in the level of transaction costs and their share in total operating costs for the first time.