## 6 Conclusions  
In this study, the nonlinear influence of built environment factors on the rail traffic flow and spatial heterogeneity characteristics in Shenzhen City were thoroughly investigated by integrating the multiscale geographically weighted regression (MGWR) model and gradient boosted decision tree (GBDT) model. The importance of built environment factors was investigated using SHAP values, and the top six key factors were identified according to their importance: office type, accessibility, road network density, building volume ratio, public services, and residential type, which, together, contribute to the total, inbound, and outbound passenger flows. It was found that the above built environment factors showed significant nonlinear effects on rail passenger flows. Analysed using PDP plots, the number of office facilities is positively correlated with passenger flow and saturates after reaching 800 while suppressing inbound passenger flow. The commuting time exceeds a threshold (25 min), and the passenger flow decreases sharply. A network density of \(\geq\)30 km/km\({}^{2}\) improves connectivity, but there is a risk of congestion between 22 and 24 km/km\({}^{2}\). The peak flows are reached at 900 for public services and 340 for residential types. Analysing the three-dimensional partial dependency plot of the combined effect of accessibility and FAR on passenger flow, combinations with FAR > 2 and accessibility < 25 min significantly attract passenger flow. In contrast, areas with FAR 1.5-3 have limited traffic growth due to congestion. These threshold effects provide a scientific basis for differentiated planning and an important guiding direction for urban transport planning. Based on this study's in-depth exploration of the nonlinear effects of built environment factors, we propose a number of transport planning policy recommendations for specific threshold effects. The specific policy advice in the context of urban transport planning and development is set out below:  
1. There is a clear threshold for the number of office facilities (800), and the efficient operation of the transport system needs to be ensured through optimal layout, staggered management, and green travel. When the number of office facilities exceeds 800, the total passenger flow tends to be saturated, and the outbound passenger flow peaks and tends to be saturated when the number of office facilities reaches 1200. Therefore, urban planners should optimise the layout of office areas to avoid traffic congestion caused by the concentration of office facilities in a certain area and reduce the pressure during peak hours by promoting demand management strategies such as telecommuting and staggered commuting, as well as encouraging green modes of travel such as bicycle sharing and walking as a supplement to ensure the sustainable development and efficient operation of the urban transport system. 2. There are key thresholds for accessibility (25 min for total passenger flow/32 min for inbound passenger flow) that need to be met through optimisation of the rail network, enhancement of hub construction, and proximity employment policies to ensure travel efficiency. Studies have shown that the psychological upper limit of commuting time in the morning peak is around 25 min; beyond which, passengers abandon metro journeys; 32 min becomes the acceptable upper limit for inbound traffic, which disappears completely after 45 min. Priority is therefore given to the development of rapid transit systems in key areas to ensure that commuting times do not exceed 25 min and to promote proximity employment policies to reduce the need for long-distance commuting. At the same time, the construction of transport hubs in suburban and peripheral residential areas should be strengthened to shorten the actual commuting time and enhance the competitiveness of rail transport, so as to ensure that residents can use public transport conveniently. 3.