Expressing contract conditions related to underlying data structures

Hello. I'm new to DbC and I'm trying to understand some of its fundamental concepts and how they could be applied to racket specifically. My questions is this: Suppose I want to define an interface. In order for the interface to be robust I want to define a contract for each method in the interface. Suppose also that in order to define some of these contracts certain statements related to the underlying data structures need to be verified. As an example, think about an interface for a train. A train is going to have train cars, by definition of train. At this point this could be modeled as an abstract contract using :exists train_cars. Now imagine that in order to define some of the contracts some condition on the train cars need to be verified and in order to verify such condition the train_cars data structure needs to be iterable(in needs to implement such interface). My question is, can I enforce such a thing in an interface? In case I can, is this a good solution to my problem or are there better alternatives?.