The Northern City Line is a line of the National Rail from Moorgate Station to Welwyn Garden City and Hertford East Stations. Unlike other National Rail lines, the Northern City is unusual in that it is partly an deep-level line running from the city to the north of London, with significant surface running sections in its outer parts into Hertfordshire. The line was formerly part of London Underground, and during its history has been part of both the Metropolitan and Northern lines, although it is connected to neither. It is now part of the National Rail network and is owned by Network Rail. The line is served by London commuter trains run by Great Northern on the lines London Moorgate - Welwyn Garden City and London Moorgate - Hertford North - (Letchworth Garden City).
It should not be confused with the City branch of the Northern line, nor with the North London Line.