The LNER Encyclopedia

This sign, on display at Liverpool Street, seems to suggest London Overground have their sights on the service between Cheshunt and Hertford East. At present they only run as far as Cheshunt, beyond there the service is provided by Greater Anglia. The clear implication from the sign though is you can get to Hertford by Overground when (for the time being at least) you can't.

Interestingly there's no London Overground symbol beside Enfield - even though it's the only station listed (apart from Liverpool Street itself) they actually serve!