In various industrial fields, the operators use predesigned procedures either to solve problems or for troubleshooting. In the Parisian subway, such procedures have existed since 1900. However, these procedures are not always exactly suited to the case at hand, and the operators generally prefer to customize a solution than to rely on fixed procedures. For the design of an intelligent assistant system aimed at supporting operators' decisions in subway control, we have modeled operators' activity and know-how. As a result, we introduce the contextual graph model, which appears as a simple solution to describe and manage operational decision making.