One of the most interesting things about them is that they provide opportunities to ride around very large cities. Large cities, and especially public transportation through large cities, are fascinating case studies in humanity. The array of building ranging from brand new to refurbished to falling down around one's ears, the shuffle of myriad people going this way and that, the various possessions that people find reason to prop up in apartment or office windows, almost everywhere you look you find something that contains a story about history or society. That's not to say that large cities are necessarily superior to their alternatives; indeed, I prefer open spaces and am firmly convinced that one can learn just as much, and live far more happily, in them than in a big city. But the metropolis has a story to tell. Much like a museum, it's full of information, but you wouldn't want to stay there once they turned out the lights.
You can tell the people who live in the city from the people who have come to visit. Those accustomed to the routine sit on the L and stare at the ground, or read, or fidget. They're tired, bored, and know how all of this works. They'll tell you where this train goes, but only if you ask. And they might not get it right. But the visitor peers out the window, hoping to learn what all the other passengers have already forgotten, as shop fronts and neighborhoods go clicking by. Who was Forrester? What did he do in that building with his name over the door? Do the architects who work in those drafting rooms now even know? If one had time and ability to get off the train and nose around, maybe one could find out. But I haven't time, and nobody else remembers even to care.
One of the particularly fascinating aspects of very large cities is the enormous number of churches. Even more excitingly, the traveler can infer that in a place like Chicago, a significant portion of them will be Catholic, and not merely the tv-dinner Protestant edifices that are stacked top to bottom in every Southern city and town. Of course, from the Metro you can only see the belfry, roof, or steeple of most of them -- sometimes one gets a glimpse of more, but not often. One inevitably finds oneself engrossed in an unwinnable game of guessing to whom each belongs. Baroque twin towers with bronze domes: surely Catholic; enormous pyramidal shingled steeple, vaguely German air: probably mainline Protestant, perhaps Prussian immigrant-built; chintzy imitation Baroque facade, no statues: American Protestant knock-off of real architecture; blocky cruciform structure with large dome over the central crossing: a long shot, but I called it for the Orthodox; Florentine-style belfry: again, surely Catholic; brown spires somewhat reminiscent of Sagrada Familia: Catholic.
Of course, without the accompanying foot tour, and never being close enough to read a sign, one is never able to verify one's score. The game ends, as does the little journey through the city, with a collection of new questions, but very few answers.