A diagnostic system for monitoring a rail system comprising a rail infrastructure and at least one fleet of rail vehicles circulating on the rail infrastructure, the diagnostic system comprising: - on-board data acquisition means comprising sensors and pre-processing means responsive to the sensors for generating rail vehicle-related sensor data representative of the operation of monitored rail vehicle components and/or of the rail vehicle environment of each rail vehicle of the fleet, - rail vehicle positioning means for generating position data representative of the position of each rail vehicle of the fleet; - rail infrastructure data acquisition means comprising sensors fixed relative to the rail infrastructure and pre-processing means responsive to the sensors for generating rail infrastructure-related sensor data representative of rail infrastructure components and/or of the rail infrastructure environment; - a database of the rail infrastructure comprising location data representative of the location of each of the sensors fixed relative to the rail infrastructure; - data processing means for merging the rail infrastructure-related sensor data, the rail vehicle-related sensor data from at least a subset of several rail vehicles of the fleet, the location data and the position data and for responsively generating series of categorized event data representative of the occurrence of categorized events at a given location on the rail infrastructure over time and/or on a given rail vehicle of the fleet over time; and - a data comparing means for comparing the series of categorized events data representative of at least one category of events over any predetermined period of time and for identifying any location of the rail infrastructure and/or any rail vehicle which exhibits a series of events data that is significantly different from the other locations of the rail infrastructure and/or rail vehicles of the fleet over said predetermined period of time. - on-board data acquisition means comprising sensors and pre-processing means responsive to the sensors for generating rail vehicle-related sensor data representative of the operation of monitored rail vehicle components and/or of the rail vehicle environment of each rail vehicle of the fleet, - rail vehicle positioning means for generating position data representative of the position of each rail vehicle of the fleet; - rail infrastructure data acquisition means comprising sensors fixed relative to the rail infrastructure and pre-processing means responsive to the sensors for generating rail infrastructure-related sensor data representative of rail infrastructure components and/or of the rail infrastructure environment; - a database of the rail infrastructure comprising location data representative of the location of each of the sensors fixed relative to the rail infrastructure; - data processing means for merging the rail infrastructure-related sensor data, the rail vehicle-related sensor data from at least a subset of several rail vehicles of the fleet, the location data and the position data and for responsively generating series of categorized event data representative of the occurrence of categorized events at a given location on the rail infrastructure over time and/or on a given rail vehicle of the fleet over time; and - a data comparing means for comparing the series of categorized events data representative of at least one category of events over any predetermined period of time and for identifying any location of the rail infrastructure and/or any rail vehicle which exhibits a series of events data that is significantly different from the other locations of the rail infrastructure and/or rail vehicles of the fleet over said predetermined period of time.