An efficient maintenance strategy is a key factor to ensure acceptable safety levels and manage the overall costs of a railway network. So far, research focused mainly on regional and high-speed rail networks, causing a knowledge and regulation gap between standard and narrow-gauge railways networks. These networks, like the local or isolated railways, have peculiar geometric and operational characteristics that play a key role on track geometry quality degradation process. For this reason, the maintenance knowledge gained in the context of standard railways when applied to systems such as narrow-gauge railways could lead to non-optimized use of the limited resources available. Therefore, the aim of this study is to analyse how the track geometry quality degradation behaviour is influenced by the key characteristics of narrow-gauge railways through the analysis of the track geometry data of an Italian local railway, laying the foundation for optimized maintenance strategies for these systems.