Transfer

one crossing joins every stop on both lines.

draw metro lines on cream paper -- where two cross, one shared station joins every stop on both, and the light floods everything it can reach

A metro map looks like a set of lines. It is really a set of crossings. On its own a line reaches only its own stations; the moment it shares a station with another line, every stop on one reaches every stop on the other, and reach stops adding and starts multiplying. That is why a network is measured by its interchanges and not by its track. Draw a line that crosses nothing and the light stops dead at both of its ends -- the same fact, seen from the side where it hurts.

Concepts: networks, connectivity, maps.

An interactive you play with one thumb, on Wundr.