Connecting cities is vital for society as well as the economy. Since the introduction of domestic flights, expectations of short travel times between city-pairs have substantially increased. However, carbon emissions from aircraft are a concern, especially as there are few options to decarbonise this sector at present. This study looks at the potential for high speed rail (HSR) to replace the busiest domestic air passenger routes worldwide and assess whether there are net carbon savings as a result. The routes are split into three categories; existing, planned and potential HSR routes. This study focuses on the potential use of air-rail partnerships on these routes to enable significant reductions in short haul domestic flights. The outcome of this research will help to inform policy makers of whether HSR is a sustainable mode of intercity transport or whether it is only viable on limited routes.