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The integration of all of those components is a different set of tests. This is not the purpose the unit tests. The unit tests allow the the supplier of those components to demonstrate the quality of the components. It is not the job of Network Rail or any of the contractors they recruit to build the infrastructure to test those components, unless there is reason to doubt the quality of the components. This wouldn’t be cost effective.
Tools like fitnesse and Gherkin and Cucumber, are an excellent way to verify what is being built (the execution of the integrated components) meets the acceptance criteria specified by the client.