This paper describes research which aims to model British rail network demand and capacity up to 2100, as part of the Infrastructure Transitions Research Consortium (ITRC). ITRC is developing models and decision support tools to enable analysis and planning of a robust national infrastructure system in an uncertain future, and the research discussed here forms part of the transport model. This is a simulation model forecasting travel within and between 142 zones, with rail traffic measured on both a link and zonal basis. The rail link model forecasts the total number of trains per track between each pair of adjacent zones, with delays acting as a brake on demand as capacity utilisation increases. Total consumption of electric and diesel fuel will also be estimated, allowing interactions with the ITRC energy model. The rail zone model forecasts the number of passengers per station within each zone, with capacity enhancements incorporated via the addition of new stations. Together these models can be used to predict rail traffic, capacity utilisation and energy consumption under a range of future scenarios, and can thus help identify which strategies for future transport infrastructure provision have the best chance of being effective in practice.