Metro Network Expansion and Passenger Demand Dynamics: A Comparative Study of Two Urban Systems

This study investigates long term passenger demand responses following metro network expansion using empirical evidence from the West Midlands Metro and Sofia Metro. Using monthly ridership data and an interrupted time series framework, the analysis evaluates both immediate and longer term effects across six expansion events. Results show that demand responses are heterogeneous across systems and events. The Sofia Line 2 extension produced a large immediate increase in ridership, while the West Midlands Edgbaston Village extension exhibited gradual post opening growth with no significant initial jump. Comparative modelling indicates that proportional immediate effects are similar across systems when scale is controlled for, but post expansion growth is significantly stronger in the West Midlands Metro. Nonlinear modelling further suggests that demand does not stabilise quickly after expansion, instead evolving over time through a ramp up process. These findings challenge conventional assumptions of rapid equilibrium adjustment and highlight the importance of system context, network maturity, and temporal dynamics in shaping demand outcomes. The study provides important implications for transport forecasting, policy evaluation, and infrastructure investment appraisal.