  This paper analyses the impact of random failure or attack on the public
transit networks of London and Paris in a comparative study. In particular we
analyze how the dysfunction or removal of sets of stations or links (rails,
roads, etc.) affects the connectivity properties within these networks. We show
how accumulating dysfunction leads to emergent phenomena that cause the
transportation system to break down as a whole. Simulating different directed
attack strategies, we find minimal strategies with high impact and identify
a-priory criteria that correlate with the resilience of these networks. To
demonstrate our approach, we choose the London and Paris public transit
networks. Our quantitative analysis is performed in the frames of the complex
network theory - a methodological tool that has emerged recently as an
interdisciplinary approach joining methods and concepts of the theory of random
graphs, percolation, and statistical physics. In conclusion we demonstrate that
taking into account cascading effects the network integrity is controlled for
both networks by less than 0.5 % of the stations i.e. 19 for Paris and 34 for
London.
