RAM apportionment model for Mass Rapid Transit Systems

Due to the rapid urbanization, rising travel needs and accessibility, Metro Rail becomes one of the preferred modes of transport for commuters. A safe and reliable Metro System with specified reliability, availability, maintainability and safety (RAMS) requirements are of prime concern. RAMS requirements can be clubbed together to derive service punctuality which can be seen as the core KPI (Key Performance Indicator of the Mass Rapid Transit Systems (MRTS). Service punctuality is tightly coupled with the RAM requirements and therefore shall be specified cautiously. Once the KPI has been specified, the next step is to decide the RAM requirements of various subsystems to meet or exceed the specified KPI target. In other words, the system punctuality target is translated in the terms of Reliability, Availability, and Maintainability at system and subsystems level. This paper presents a top-down RAM apportionment model for generic MRTS systems. The proposed model endorses the system level KPI by translating and apportioning the system level RAM requirements into requirements for each subsystem, equipment or component. As a case study, the proposed model has been applied to one of key Metro Rail Project and its various subsystems in India. Methods for deriving mean time between failures (MTBF), mean time to repair (MTTR) and % availability of all systems are also discussed.