la ligne 4 coupe toutes les autres

In the Paris metro network, one single line manages to intersect ALL the other ones -line 4, from Clignancourt to Montrouge.
So, in theory, you can get from any place to any other one with at most two line changes. Not a good idea most of the time in fact as there are faster paths, but a mathematician may appreciate ;-)
In general, in the wagons, aside the line schematics you have a brief summary of the intersected lines, but for the 4 you have, well, the whole list, at right...