the subway reduces
to a question of mass transport
along a directed graph
with around a hundred nodes, or stations.
of course the weights on the arcs
(representing the number of passengers traveling
between each two stations)
does not remain constant
so we must introduce time,
i.e, a differential equation, but the problem remains,
nonetheless, trivial.
the solution to the rush hour crunch does require fractional trains,
naturally, but that is not a problem for the theoretician.
