A number of selected field-study techniques have been validated in a case study in the domain of railway signal boxes. The context of this work is the endeavour of a human-centred design consultancy to acquire know-how on HCI methods for use in the design of safety-critical systems. The field studies were aimed at providing a clear overview of the work environment, tasks and cognitive load of the signallers and of possible bottlenecks in the current way of operating. Our approach for conducting field studies in supervisory control systems is based on ethnomethodology, situation awareness, and mental models ted as an abstraction hierarchy. For each of these methods, we discuss our approach, the result, and the applicability of the technique to future safetycritical system design projects. Keywords. Safety-critical systems, ethnomethodology, patterns of cooperative interaction, situation awareness, goal-directed task analysis, mental models, ion hierarchy