The quality of a public transport network is not only determined by its performance in non-disturbed situations. On a quite frequent basis public transport service do not operate according to plan. This urges for analysis on how well a network is capable to perform in such situations. How well is a PT network capable of maintaining its network performance in case of disruptions? Or in other words, how robust are PT networks? To answer these questions an assignment model is developed and applied to the Amsterdam tram and metro network. The model calculates the performance of the network in case of local capacity reduction. Based on these results the influence of link characteristics on the criticality of links and the degrading rapidity are estimated.