Network robustness refers to as the capacity to absorb disturbances with a minimal impact on system performance. Notwithstanding, network robustness assessment has been mostly confined to the analysis of complete link breakdown based on topological metrics. We propose reliability indicators that encompass changes in network performance with respect to the entire range of possible capacity reductions. Link criticality and degradation rapidity are measured by constructing network degradation curve that describe the relation between local capacity reduction and global change in network performance. We develop a public transport robustness assessment model which computes passenger flow distribution and network performance metrics under planned capacity reductions. The model is applied to the urban rail-bound network of Amsterdam. Link criticality and degradation rapidity are studied by performing a full-scan impact analysis which demonstrates how the robustness indicators introduced in this paper contribute to a more complete assessment of network robustness.