  The behavior of complex networks under attack depends strongly on the
specific attack scenario. Of special interest are scale-free networks, which
are usually seen as robust under random failure or attack but appear to be
especially vulnerable to targeted attacks. In a recent study of public
transport networks of 14 major cities of the world we have shown that these
networks may exhibit scale-free behaviour [Physica A 380, 585 (2007)]. Our
further analysis, subject of this report, focuses on the effects that defunct
or removed nodes have on the properties of public transport networks.
Simulating different attack strategies we elaborate vulnerability criteria that
allow to find minimal strategies with high impact on these systems.
