eeerrr, sorry, it does what is says on the tin surely..??
ie, goes overground, is a part TfL's network along with london UNDERground, & is separate from national rail providers.. which also go overground but are distinct.
& of course rail tickets can be passed through & recognised by the turnstiles as usual.
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BnV types in lower case - get over it - this by no means affects articulation... love ya.