In the public transportation system, punctuality benefits both bus operation and passengers’ travel experience. However, uncertainty exists due to complex traffic conditions and heterogeneous driving behaviors. To analyze bus operational uncertainty, transport planners and bus operators need a tool that supports multi-granular modeling, spatio-temporal representation, and interactive exploration. To meet the requirement, we present MetroBUX, a visual analytics system for $B$ us operational $U$ ncertainty e $X$ ploration. MetroBUX aligns daily bus trips and models stop-level uncertainty of bus arrival time. It has a consolidated interface with three main views: Map View for presenting the spatial distribution of uncertainty, Temporal View for tracking the evolution of uncertainty, and Trip View for inspecting uncertainty propagation. Specifically, MetroBUX enables integrated spatio-temporal analysis by connecting topological uncertainty distribution at different periods in a nested tracking graph. Furthermore, it supports interactive and hierarchical exploration, including region-, route-, trip-, and stop-level analysis. Case studies on real-world bus operational data and domain experts’ feedback demonstrate the efficiency of MetroBUX.