Abstract Background In Australia, an estimated 57% of the population do not meet physical activity recommendations for health. The built environment is important for active living, and recreational trails provide safe and pleasant settings for this purpose. However, evidence for positive impacts on physical activity from real world natural experiments is sparse. We describe the impact of transforming a recreational trail into a loop on usage by cyclists and pedestrians and users’ physical activity levels. Method We conducted time series analyses of pre and post-completion (November 2013–July 2015) counts taken from infrared electronic counters of pedestrians and cyclists on two established sections of the trail adjusted for underlying trend, trend change, weather, holidays and trail closures. Chi-square analyses of pre and post-completion visual counts examined change in the distribution of pedestrian/cyclist, adult/child, and male/female users. Descriptive and bivariate analyses of post-completion intercept survey data of 249 trail users were conducted to examine user characteristics and impact on physical activity. Results Pedestrian and cyclist counts on established trail sections increased by between 200 and 340% from pre to post-completion. Visual count data showed a significant 7% increase in children (vs adults) using the trail at one site pre to post (p = 0.008). Of previous users, 48% reported doing more physical activity at the trail and this was additional to (not replacing) physical activity done elsewhere. Those users not meeting physical activity recommendations were more likely to report increased total physical activity since the loop was created (55.5% vs 39.2%, p = 0.031). The connected loop nature of the trail and its length was perceived to encourage more and different forms of physical activity. Conclusion Creating an accessible loop trail away from motorised traffic can lead to increased trail use and potentially total physical activity. The modification to the trail encouraged proportionate and real increases in usage among vulnerable populations such as children and perhaps greater total physical activity especially for people not meeting physical activity recommendations. The findings suggest that the benefits of environmental changes such as these can accrue to those most in need of support for being physically active.