Expansive learning processes are increasingly often studied and facilitated by interventions in multi-organizational terrains of object-oriented activity. Such terrains are occupied by multiple activity systems which commonly do not collaborate very well although there are pressing societal needs for such collaboration. In such divided terrains, expansive learning needs to take shape as renegotiation and reorganization of collaborative relations and practices between and within the activity systems involved. This is radically different from traditional workplace learning which mainly consists of workers becoming competent or improving their competencies within the established practices and along the established measures of their own activity systems. Standard notions of workplace learning cherish a vertical view of competence and expertise. Characteristic to this view is a discourse of 'stages' or 'levels' of knowledge and skill. Such a vertical image assumes a uniform, singular model of what counts as an 'expert' in a given field. However, the world of work is is increasingly organized in ways that require horizontal movement and boundary crossing. Such horizontal or sideways movement needs to be conceptualized and modeled on its own terms. Adrian Cussins' theory of cognitive trails offers a promising vocabulary and model for depicting and analyzing this movement. In the paper, the theory of expansive learning and the theory of cognitive trails are brought together to develop and test a methodology for analyzing learning in encounters between medical professionals representing different caregiver organizations in Helsinki, Finland.