The influence of rail cant on the overall geometry conditions of railway tracks is assessed both theoretically and experimentally in this study. Extensive data obtained from a light railway line are used for experimental investigations. Modelling of rail—wheel contact using the ADAMS/Rail program is undertaken for theoretical investigations. The need to incorporate rail cant into the development of the track geometry indices is identified, and a method of performing this is proposed. Using this approach, a new track index is defined, for which the main geometry parameters, including alignment, profile, track twist, track gauge, and rail cant (rail twist), are considered. For the development of the new track index, these parameters are combined, assigning justified coefficients to each geometry parameter according to its contribution to the overall conditions of the track. The research methodology and the results are discussed. A practical use of the new index is presented to indicate its capability and applicability.