A public passenger transit system is complex, because it involves two types of traffic unit, passengers and vehicles. The system operations require supplying passengers with stations where they can access vehicles and then travel in them between stations. These operations are subject to various capacity constraints, depending on the traffic type and the situation in the system. The article provides a framework for the analysis of a mass transit system, broken down into four subsystems that respectively pertain to the Passenger, the Vehicle, the Station and the Line. Each subsystem consists of several components that are involved in the system processes and interactions. Within this framework, capacity phenomena are identified, described qualitatively and classified into seven broad categories, namely: (i) the vehicle capacity of an infrastructure; (ii) the vehicle fleet; (iii) the passenger capacity of a vehicle; (iv) the passenger capacity of a route; (v) the passenger capacity of a station; (vi) the vehicle storage and movement capacity of a station; (vii) the capacity of a station for interface with personal transport modes. Moreover, it is shown that capacity phenomena can interact in congestion gears, which impair capacity. Lastly, there are complex retroactive effects that involve either network management or demand behaviour (through route choice which depends on quality of service hence on congestion).