Service frequency variability and its dependence on the passenger number and fleet composition

Rail and metro systems are generally considered uncongested, as variations in running times between stations are negligible compared to other systems, such as road transport, where congestion can increase travel times more than three times compared to normal conditions. This study analyses the effects of variability in the number of passengers on rail and metro vehicles (and related variation in total train weight) on running times. Indeed, although the variations in running times are neglectable, they can compromise the stability of service frequency, triggering mechanisms that lead to a deterioration of system performance. Therefore, the paper proposes a methodology for analysis to assess whether such variability could influence the stability of service frequency. An application to Line 1 of Naples metro systems, in Italy, has shown that in ordinary conditions the layover time is able to absorb any running time variations thus ensuring the stability of the service frequency.