<p>Equitable public transport (PT) provision requires balancing often competing principles: horizontal equity, serving population demand, and vertical equity, prioritizing social needs. This paper introduces diagonal equity, a novel metric integrating both dimensions into a unified framework. Using the Metropolitan Area of Porto as a case study, walking access to PT stations was evaluated alongside population demand and social needs at the census-block level using principal component analysis of demographic and socioeconomic variables. Horizontal and vertical equity were assessed through accessibility gaps relative to demand and needs, while diagonal equity was evaluated using the potential captive demand, accounting for both. Results show that potential demand is concentrated in central municipalities, whereas social needs are dispersed across suburban and rural areas. Diagonal equity identifies disadvantaged areas where access fails to serve demand-needs, providing a more nuanced view than individual equity assessments and a practical decision-support tool for inclusive PT planning and design.</p>