When two lines cross, a cello note is plucked (and the theretofore taut animated line makes a cute little vibro-wobble); the pitch of the note is determined by the length of the crossing line. "A complete chromatic scale was too dissonant," writes Chen. "Ultimately I settled on a simple major C scale but with the lowest note as a raised third E, which keeps it from ever feeling fully resolved."
Predictably, the cluster of notes starts to pick up steam around the morning and evening rush hours, although there are clumsy ways to bend the instrument to your will. (Pro tip: keep it open in at least two windows.) And look out for little touches like the ghost trains -- lines drawn by routes that the MTA has discontinued since the 1972 illustration on which Chen's graphical interface is based -- that run between midnight and 2 a.m.
So how 'bout it, S.F. Muniphiles -- where's the left coast equivalent?