A well-functioning public transport network is the foundation of sustainable urban mobility. However, the term 'well working' is difficult to define and may be subjective. This paper introduces a network-analytical approach to show the correlation between usage, which reflects the quality of a public transport service, and the structure of its network. This approach is based on Barabási's definition of a scale-free network and physicists' observations of network structures. Ultimately, the paper will show that scale-free public transport networks are more efficient in terms of usage, as modelled by the modal split of the observed cities.