In conventional railway systems (or fixed-block railway signaling systems), railway lines are divided into subsections, called railway blocks, which start and end with a signal. In order to prevent collisions, only one train is allowed in each railway block at a time. Since the occupancy of the next block is indicated by the signals, train drivers have to pay attention to the signals during their journey. In spite of the conventional railway systems having several drawbacks such as the reduction in railway line capacity and the same safe braking distances for all kinds of trains, they have been in use since the mid-1800s. In this paper, we study fault diagnosis in fixed-block railway signaling systems from the discrete event systems point of view: first the signaling system equipment are modeled by using Petri nets, and next a diagnoser is designed to show the diagnosability of the system.