An interactive hybrid computer technique which facilitates the design of a fixed block signaling system for a metro network is described. The signaling philosophy is the three-aspect three-block cab signal system employed by the majority of metro networks throughout the world. Two constraints are of critical importane for such a signaling philsophy: the stopping distances (which determine minimum block lengths), and the throughput (which indirectly governs maximum block lengths). Taking into account these two constraints, the hybrid computer technique described in this paper permits the design engineer to interactively choose the block lengths and locations. The technique may be used to design the signaling for the new Montreal Metro line currently under constructon.