Hertford loop line

The Hertford Line (also known colloquially as the Hertford Loop) is a branch of the East Coast Main Line, part of the Northern City Line commuter route to London for Hertford and other Hertfordshire towns and an occasional diversion route for the main line. The line is part of the Network Rail Strategic Route 8, SRS 08.03 and is classified as a London and South East Commuter line.
History.
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 Welwyn Viaduct. Work started in 1905 and Cuffley was reached on 4 April 1910. The construction of two major viaducts and the Ponsbourne Tunnel (at , 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 Hertford North Station was opened. The line was electrified in 1977.
The line was also used frequently during the Second World War as the Welwyn/Digswell viaduct was at high risk from bombs
Route and settlements served.
The Hertford Line leaves the East Coast Main Line at Langley junction, just south of Stevenage.
It rejoins the East Coast Main Line at Wood Green South junction, north of Alexandra Palace railway station.
Operations.
Great Northern operates suburban services along the Hertford Line between London King's Cross or Moorgate, and Stevenage, Watton-at-Stone or Hertford North. Occasionally, London North Eastern Railway, Hull Trains, Grand Central 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 engineering work. There is a reversing siding to the north of Bowes Park which is occasionally used to reverse London North Eastern Railway trains heading for the Bounds Green Depot. There are also bay platforms at Hertford North, Stevenage, and Gordon Hill, the latter acting as a terminus during peak hours and night only.
Infrastructure.
The line is about 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 .
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 Line include the following.
ERTMS trials on the Hertford loop line.
Network Rail is using Beacon Rail owned Class 313 unit 313121 as a test vehicle for ERTMS on the Hertford Line. The plan involved resignalling a 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.
References.
Hornby, Frank (1995) "London Commuter Lines. Volume 1: Main lines north of the Thames". Kettering: Silver Link Publishing Ltd.