def get_travel_requests_of_timetables(timetables):
    """
    Retrieve a list containing all the travel_request_documents,
    which are included in a list of timetable_documents.

    :param timetables: [timetable_documents]
    :return: travel_requests: [travel_request_documents]
    """
    travel_requests = []

    for timetable in timetables:
        travel_requests_of_timetable = timetable.get('travel_requests')
        travel_requests.extend(travel_requests_of_timetable)

    return travel_requests