Final answer:
The XX key on a metro key ring is likely a universal key for a particular transportation system, used to open doors designated for emergency or service access by authorized personnel.
Step-by-step explanation:
The answer to the question regarding which doors can be opened from the exterior by using the XX key found in the metro key ring depends on the specific transit authority or organization's key management system. The XX key is likely a universal key designed for use within a particular public transportation system.
The doors that this key opens are usually restricted to emergency or service access, such as control panels, service gates, or maintenance doors. Only authorized personnel would typically have access to such a key. For an accurate list, one would need to refer to the transit authority's access control documentation or policies.