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What is the cardinality of this relationship? is m-n cardinality correct? if not, why?

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Yes cardinality of relationship travels is M:N where one entity is participating totally and another is participating partially.
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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.

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Yes the cardinality is m:n is correct.On the passengers side the participation is mandatory(total) while on the train side its partial

NOTE:While converting to ER-Model the number of relations will be 3 with a junction table (considered as best design) and derived attribute wont be stored as it can be computed from the stored or the base attribute

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