  What if $\{$a tourist, a train addict, Dr. Sheldon Cooper, somebody who likes
to waste time$\}$ wants to visit all metro lines or carriages in a given
network in a minimum number of steps? We study this problem with an application
to the metro network of Paris and Tokyo, proposing optimal solutions thanks to
mathematical programming tools. Quite surprisingly, it appears that you can
visit all 16 Parisian metro lines in only 26 steps (we denote by a step the act
of taking the metro from one station to an adjacent one). Perhaps even more
surprisingly, adding the 5 RER lines to these 16 lines does not increase the
size of the best solution. It is also possible to visit the 13 lines of (the
dense network of) Tokyo with only 15 steps.
