Since the last years, the electro-mechanical systems for route-setting are being replaced worldwide with computer-based interlocking system. The replacement typically requires significant interventions for substituting existing electro-mechanical systems up to the railway-yard where the point switches, light signals, track circuits, etc., remain unchanged. The significant changes, allow a significant simplification of the connection network between devices and control apparatuses thus making communication more reliable and processing nucleus dimensions drastically reduced. However, the resulting overall complexity of computer-based procedures makes necessary analyses on the system as a whole. Therefore a reliability analysis of the new system seems essential jointly with a right choice of modelling methods. The present paper illustrates the reliability and availability analyses performed for the new computerised interlocking system implemented for controlling the Roma Termini railway station traffic.