  Metros (heavy rail transit systems) are integral parts of urban
transportation systems. Failures in their operations can have serious impacts
on urban mobility, and measuring their robustness is therefore critical.
Moreover, as physical networks, metros can be viewed as network topological
entities, and as such they possess measurable network properties. In this
paper, by using network science and graph theoretical concepts, we investigate
both theoretical and experimental robustness metrics (i.e., the robustness
indicator, the effective graph conductance, and the critical thresholds) and
their performance in quantifying the robustness of metro networks under random
failures or targeted attacks. We find that the theoretical metrics quantify
different aspects of the robustness of metro networks. In particular, the
robustness indicator captures the number of alternative paths and the effective
graph conductance focuses on the length of each path. Moreover, the high
positive correlation between the theoretical metrics and experimental metrics
and the negative correlation within the theoretical metrics provide significant
insights for planners to design more robust system while accommodating for
transit specificities (e.g., alternative paths, fast transferring).
