  The behaviour 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 a recent study of public transport networks
of 14 major cities of the world we have shown that these systems when
represented by appropriate graphs may exhibit scale-free behaviour. In this
paper we briefly review some of the recent results about 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.
