The article contributes to understanding public transport as a public space by exploring diversity and the city through mobility. It investigates the compressed and mobile space of the 90/91 trolleybus in Milan. Due to its itinerary and extended schedule, this bus is intensively used by citizens with different ethnic, economic, social and cultural backgrounds. Literature on planning and transport has recently started exploring qualitative issues through individual ethnographic research on transport means. Research on everyday multiculturalism, despite recognising the role of public transport as a promising space to study the negotiation of difference, rarely adopts this specific focus and does it mainly from a socio-anthropological point of view. Against this background, the work investigates the compressed space of a bus through an ethnographic exploration of people, spaces and practices onboard. Notably, the article is is grounded on direct observation carried out by three classes of students in the Urban Ethnography course offered in the MSc in Urban Planning and Architecture at Politecnico di Milano, and presents a post-hoc reflection on the outcomes of the teaching project. Grounding on this experience, the article argues that the compressed and mobile space of public transport is an excellent observation point to investigate everyday negotiation of difference and a privileged observatory of broader city dynamics. Additionally, the multiplication of points of view embedded in the observations and experiences of students has proved how, in the face of increasingly diverse cities, pluralisation may be a key methodological approach.