This paper investigates the effectiveness of Duration Calculus in the specification of safety, as well as, of functional requirements of critical systems. It is based on railway signalling - a case study which is realistic both in terms of complexity and in terms of the range of safety requirements covered by it. Duration Calculus is found to be a convenient medium of specification, firstly, due to its reliance on durations of states rather than on real time in explicit form and, secondly, due to the intuitive nature of its syntactic constructs.