This study looks at children's music making in a public setting designed for society writ large. Although known to most children in the city, the subway presents a unique environment, both structurally predictable yet sonically dynamic; it is in continuous transition. Adopting Winnicott's psychoanalytical perspective, we make a case for viewing children's spontaneous music making as a tool for comfort and engagement, specifically through the use of musical material as transitional objects and musical behaviors as transitional practices. Data were collected over three weekends on two subway lines in New York City. Seven to 10 field observers traveled in groups of two or three; each filled out a protocol form for every episode of children's music making observed, indicating musical qualities, sources, and contexts. These categorical descriptors and accompanying field notes were later used to construct more detailed qualitative narratives. With a total of 69 episodes, the authors found that music making was present in children from infancy to middle childhood (approximately age 10). Findings showed musical behaviors were influenced by adult interaction, and generated by resources in the environment. Over 81% of observed episodes contained vocal behaviors; movement occurred in almost half (48%). Musical materials were mostly invented. The most frequent function was "to comfort and entertain self," comprising over 50% of those observed, and was linked to solitary interaction. Communicating with others accounted for another third of the reports.