The crucial context for many of the 1970s studies was the Queens murder of Kitty Genovese, whose cries for help were purportedly ignored by her neighbors. The Genevose story became the ur-narrative of uncaring urban pathology (even if its details were later called into question). The subway offered a perfect testing ground for the emerging subfield of "bystander studies." The aforementioned "Good Samaritan" paper, for example, had a Columbia University student stagger and collapse on a subway train, "looking supine at the ceiling." In some trials, the subject acted drunk; in others, ill. (People were more likely to help in the latter condition.) Interestingly, that study found no support for the so-called "diffusion of responsibility" effect—the idea, per the Genovese murder, that the more bystanders were present, the less likely it was that any one person would help. In fact, the reverse was found.
Another Milgram classic was the "lost-letter technique": Approach a stranger with a letter (sometimes stamped, sometimes not) and ask her to mail it for you. The number of mailed letters provides the success rate. One 1975 study by psychologist Roy Feldman stationed experimenters (both locals and foreigners) in the subway stations of several cities (Boston, Athens, Paris) and had them ask the postal favor. One interesting (if limited) finding: Greeks were less likely to mail a letter for fellow Greeks than for a foreigner.
One of the key tenets of "civil inattention" is the scrupulous avoidance of direct eye contact, and a number of studies have examined this variable. In one 1974 study, experimenters actively stared at people, then asked for help. Subjects were more likely to help when the person hadn't been staring (echoing a long line of studies finding that people generally become uncomfortable in the face of an unprovoked or inappropriate gaze—unless, of course, the gazer is an attractive woman). Another paper, by Clark McCauley, et al., published in 1978 in Environmental Psychology and Nonverbal Behavior, looked at the overall willingness to make eye contact in a commuter train setting in a city environment (Philadelphia) and its suburbs. Commuters were more gaze-shy in the city (only 13 percent of passersby were willing in Philly, as opposed to 31 percent in Bryn Mawr). The authors attributed the results not to rudeness but to the overabundance of information available to process—as they put it, "interpersonal overload leads to social withdrawal."
Of course, a passenger's responses may depend on how long he's been on the train: A study by Gary Evans, et al., linked the length of the commute to stress levels. The longer the voyage, the higher the levels of cortisol in the saliva of test subjects, and the worse they did on a "performance task" (proofreading). Other studies have shown that simply not having to make a transfer reduces stress.
Crowding levels matter too, though not necessarily in the way you might think. In another study by Evans and Richard Wener, higher stress levels were linked less to overall subway car crowding than to how densely the seats were occupied. More crowded trains also increase the chance of physical contact; a study by David Maines looked at seating position (specifically, elbows: "Elbow manipulation also becomes one way of expressing sentiment concerning the person sitting next to you") and hand position on the straps, and found that incidental touching was more likely to occur among same-race, same-sex passengers. "Race and sex redefine physical distance," he wrote. (With gender, it's the lack of this distance, of course, that has led to "women-only" subway cars, from Mexico City to Mumbai.)
Has the era of the electronically immersed commuter, oblivious to all around him, lost in his playlists, signaled a decline in subway psychology? Not that I can find. One recent study conducted by officials at the Paris Metro—which looked at "missed connection" ads placed by urbanites looking for love in the city—found that the Metro "is without doubt the foremost producer of urban tales about falling in love." The seats closest to the door, it seemed, offered the best opportunities for falling in love with the proper stranger. "The Metro is not the emotional desert, the social vacuum, that we sometimes believe it to be," observed the chief of the Paris Metro. Indeed, whether bystanders or participants, we're all part of an ongoing experiment.