The control of the subway line traffic is a domain where operators must deal with huge quantities of pieces of knowledge more or less implicit in the control itself. When an incident occurs on a subway line, the operator must choose the best strategy applicable for moving from the incidental context to the operational one. An incident on the subway line may cause traffic delay or service interruption and may last for a long or short time, depending on the nature of the incident and many other elements. Operators mainly focus on contextual information for incident solving. An operator said, “When an incident occurs, I look first at what the incident context is”. We propose to support subway line traffic operators in incident-solving with an incident-manager system, which is a part of the SART project (French acronym for support system in the traffic control). The incident manager is a decision-support system based on the contextual analysis of events that arise at the time of the incident. It uses a context-based representation of incidents and applies a context-based reasoning. In this paper we discuss a context-based representation of incidents on the basis of the onion metaphor. The SART project now enters the second year of the system design and development and implies two universities and two subway companies in France and Brazil.