  The behavior of complex networks under failure or attack depends strongly on
the specific scenario. Of special interest are scale-free networks, which are
usually seen as robust under random failure but appear to be especially
vulnerable to targeted attacks. In recent studies of public transport networks
of fourteen major cities of the world it was shown that these systems when
represented by appropriate graphs may exhibit scale-free behavior [C. von
Ferber et al., Physica A 380, 585 (2007), Eur. Phys. J. B 68, 261 (2009)]. Our
present analysis, focuses on the effects that defunct or removed nodes have on
the properties of public transport networks. Simulating different directed
attack strategies, we derive vulnerability criteria that result in minimal
strategies with high impact on these systems.
