This paper is aimed at presenting a service formation process in a transport information system which is dedicated to the co-modal transportation service. The service formation is achieved via the coalition formation to get a full itinerary with well-assigned route segments. Meanwhile, the coalition formation targets to achieve the optimum combination with respect to the user's preferences. The multi-section characteristics (with transfers) of the journey and numerous simultaneous requests make it necessary to apply a multi-agent approach. With the role-switching of the agents, the agent behaviors and protocol are established to normalize every step of the agent coalition.