Integrating soil-structure interaction and intelligent critical speed control for monorail networks: Design charts for enhancing the sustainability of autonomous trains

• Design–oriented decision-support framework for CS evaluation in straddle-type monorails. • 3D validated nonlinear FE model coupling soil–bridge–train interaction using the HSsmall soil constitutive law. • Precomputed design charts for rule-based estimation and avoidance of resonance-related CSs. • Bidirectional optimization of bridge spans and train geometry to to reduce vibration-induced maintenance demands. • Conceptual and future-oriented adaptive bogie spacing strategy enabled by critical-speed-aware design tools. The widespread adoption of straddle-type monorails for sustainable urban transport necessitates careful consideration of the critical speed (CS) of the soil–bridge–train system, at which resonance-induced vibrations may significantly amplify structural and vehicle responses. Operating near this speed range can accelerate damage to bogie components and increase maintenance demands. Therefore, reliable identification and avoidance of CS is essential for safe and cost-effective monorail operation. This study presents a comprehensive framework for evaluating the CS of straddle-type monorail systems by integrating soil–structure interaction effects with detailed bridge and train modeling. A validated three-dimensional nonlinear finite element model, incorporating inhomogeneous soil behavior and hysteretic damping through the HSsmall constitutive model, is employed to conduct an extensive parametric investigation. The analysis examines the influence of soil conditions, bridge geometry, and train configuration, including bogie spacing, axle loads, train length, and single- and double-line crossings. The primary outcome of this study is a set of precomputed design charts that enable rule-based CS estimation and management, allowing engineers to identify resonance-prone speed ranges without resorting to repeated large-scale numerical simulations. These charts support bidirectional design optimization of both bridge spans and train geometry, facilitating the desynchronization of operational speeds from critical resonance conditions. By enabling systematic avoidance of critical speeds, the proposed framework contributes to reduced vibration-induced deterioration of bogies and suspension components, leading to lower maintenance requirements, extended service life, and improved operational reliability. These engineering-level benefits indirectly support the economic and environmental sustainability of urban monorail transportation systems.