The line you refer to is the Hatfield, Luton & Dunstable branch of the Great Northern Railway. It dates back to the 1850s and was built by an independent company funded by Luton businessmen and local landowners. It was later absorbed by the GN, LNER, then BR Eastern Region.
From Hatfield the line ran parallel to the GN main line as far as Welwyn Junction (more or less where WGC station is now) where the branch diverged and ran to Ayot, Wheathampstead, Harpenden, Luton, Dunstable. From Harpenden to Luton it ran parallel to the Midland (constructed a few years later), but there was no connection. The Midland had a branch from Harpenden to Hemel Hempstead and Boxmoor and that is the line that was called the Nicky line. Again no physical connection to the Welwyn Line, the stations in Harpenden being about half a mile apart and separated by a large ridge. At Dunstable the line met the LNWR branch from Leighton Buzzard.
There were branch passenger trains and through trains to London in the early days. Also a lot of goods traffic from Luton and the upper Lea Valley. It closed to passengers in 1965 and completely in 1971. The line was permanently severed at Ayot by the building of the A1(M) a year or two later and the rails must have been lifted in the late 70s.
I remember seeing or have photographs of the following diesel services: Class 08, 15, 26, 31, Cravens 105 DMU. In earlier days I believe the branch was worked by tank engines such as N7 based at Hatfield, maybe also N2, N7 etc.
I suggest you try the book by Sue and Geoff Woodward for more info. The second edition (or later) is better. They also run the private Harpenden Railway Museum, well worth a visit if you are passing, has a lot about the line.
Some info and photos at
http://www.disused-stations.org.uk/l/lu ... ndex.shtml and also Wikipedia.
Simon.
