This thesis addresses a hypothetical scenario in which all flights within most of Europe are substituted by rail and estimates the resulting additional rail demand, with a focus on night train demand. Routes, cities and countries with high potential for (night) trains under maximum air-rail substitution are highlighted, and rolling stock requirements are estimated. Several research gaps regarding long-distance travel behaviour and mode choice are shown to require assumptions in the modelling approach; the results are indicative of areas with higher demand potential for night trains than others.