Hertford Loop line

The line was opened in three stages between 1871 and 1924. The first section called the Enfield Branch Railway was developed by the London and York Railway and went from Wood Green to Enfield. In 1898, a plan was approved to extend the line north to Hertford and Stevenage, in order to relieve congestion on the main line without having to widen the Digswell Viaduct. Work started in 1905 and Cuffley was reached on 4 April 1910. The construction of two major viaducts and the Ponsbourne Tunnel (at 2,684 yards or 2,454 metres, the longest in the eastern counties of England and the last to be built by traditional methods), combined with World War I shortages of men and materials, delayed the opening of the route to Stevenage until 4 March 1918. Then it was single track and for goods services only. The line finally opened to passengers on 2 June 1924 when a new station at Hertford North was opened. The line was electrified in 1977.

The line was also used frequently during the Second World War as the Digswell Viaduct was at high risk from bombs.

The Hertford Loop line leaves the East Coast Main Line at Wood Green South Junction, north of Alexandra Palace. It then serves:

• Bowes Park, serving Bowes Park, Bounds Green and Wood Green
• Palmers Green, serving Palmers Green
• Winchmore Hill, serving Winchmore Hill
• Grange Park, serving Grange Park
• Enfield Chase, serving Enfield Town
• Gordon Hill, serving Gordon Hill and Forty Hill, also close to Chase Farm Hospital.
• Crews Hill, serving the village of Crews Hill
• Cuffley, serving the villages of Cuffley, Northaw and Goffs Oak
• Bayford, serving the villages of Bayford and Brickendon
• Hertford North, serving the town of Hertford
• Watton-at-Stone, serving the village of Watton-at-Stone

The line then rejoins the East Coast Main Line at Langley Junction, just south of Stevenage.

Great Northern (part of Greater Thameslink Railway) operates suburban services along the Hertford Loop line between London King's Cross or Moorgate, and Stevenage, Watton-at-Stone or Hertford North. There are also bay platforms at Hertford North, Stevenage, and Gordon Hill, the latter acting as a terminus during peak hours and night only.

Occasionally, London North Eastern Railway, Hull Trains, Grand Central, Lumo, Thameslink and other faster Great Northern services operate non-stop along the route when diverted off the main section of the East Coast Main Line, due to incidents or planned engineering work. There is a reversing siding to the north of Bowes Park which is used to reverse London North Eastern Railway trains heading for Bounds Green Depot.

The line is about 24 miles (39 km) long, is double track throughout and is electrified at 25 kV AC using overhead line equipment. It has a loading gauge of W9 and a maximum line speed of 75 mph (121 km/h).

Grade-separated junctions connect each end of the northbound track with the main line. All stations are long enough to accept two three-car (20m) EMUs. Not all stations are long enough for newly introduced 6-car EMUs, but their walk-through design allows for access.

Tunnels and viaducts

Major civil engineering structures on the Hertford Loop line include the following.

Network Rail used Beacon Rail owned Class 313 unit 313121 as a test vehicle for ERTMS on the Hertford Loop line. The plan involved resignalling a 5+1⁄2-mile (8.9 km) section of the double track route to allow existing passenger and freight services to work bi-directionally over the up Hertford Loop line, freeing the down line for ERTMS tests and evaluation.

Takeover by TfL

On 20 September 2025, Transport for London (TfL) submitted an outline business case to take over Great Northern services from Moorgate to Welwyn Garden City, Hertford North, and Stevenage and integrate it into the London Overground network. TfL wants to increase the number of trains from two trains per hour to four trains per hour, standardise fares across the route, serve a 21,000 home development at Crews Hill, and boost growth in North London and Hertfordshire. The takeover could occur in Autumn 2027, after Govia Thameslink Railway's franchise ends, if approval is given by DfT; however, the Overground services would use the existing Class 717 stock.