The participation of an entity set E in a relationship set R is said to be total if every entity in E must participate in at least one relationship in R. If some entities in E do not participate in relationships in R, the participation of entity set E in relationship R is said to be partial.
In this example, PASSENGERS has total participation, and TRAIN has partial participation. That means every passenger must travel at least one train, and every train may or may not contain passengers. So, it is clear that the cardinality of the above diagram is M:N.