It is part of the Great Northern Route services, and operates as the south-eastern branch of the East Coast Main Line (ECML).

Built as an isolated route with a northern terminus at Finsbury Park, reconstruction connected it to the British Rail network in 1976 and began its modern service pattern.

The Great Northern & City Railway (GN&CR) was planned to allow electrified trains to run from the Great Northern Railway (GNR, now the East Coast Main Line) at Finsbury Park to the City of London at Moorgate.

[9] During this period, the line remained an isolated branch, without through services to any other part of the rail network.

Carriages were brought to it through a connection into a freight yard near Drayton Park station, where a small depot was built to service trains.

As part of London Transport's New Works Programme, the Northern Heights plan was to connect the Northern City Line at Finsbury Park to existing main-line suburban branches running to Alexandra Palace, High Barnet and Edgware, which would be taken over by London Transport and electrified.

The lines would have run in small-diameter tube tunnels south from Moorgate to Bank and London Bridge.

In 1970 the line was connected (as intended by its original promoters) to the mainline via the high level platforms at Finsbury Park as part of a wider plan to electrify ECML suburban services.

There are two electrical sections,[11] separated by a gap at Poole Street: Trains change from AC to DC traction supply, or vice versa, whilst standing at Drayton Park station.

In the 1970s the original London Underground fourth-rail traction current supply was converted to a third-rail 750V DC system.

The redundant centre fourth-rail was disconnected and left in situ since it was difficult to remove in the narrow tunnels.

[11] Works to commission new ETCS digital signalling on the line, along with the first test train under the new system, took place during the Early May Bank Holiday in 2022.

[13] The new signalling system was approved by the Office of Rail and Road in March 2023, and gradually introduced on passenger services from 27 November 2023.

[16] Because mainline trains operate over the infrastructure inherited from London Underground, there are some practices on the NCL which differ from Railway Rulebook instructions, and these are contained in an additional publication.

1938 tube stock serviced the line, in variously three-, four- and six-car formations, until its temporary closure on 4 October 1975.

[20] To comply with regulations for trains operating in single-bore tunnels where there is not enough clearance space for side evacuation, they have emergency doors at both ends of a unit.

All of the Class 313 units when operating on the NCL had their Driving Motor B vehicle at the London end, and whilst on 750 V DC were electronically limited to 30 mph (48 km/h),[21] which is the maximum line speed.

A report found that there was insufficient evidence to say if it was a deliberate act of the driver or due to a medical condition.

A serious accident was averted by the actions of an observant train driver, and the line was restricted for several days for repairs.