The former trackbed has been redeveloped as a rail trail and is part of the Oxford to Welwyn Garden City route of the National Cycle Network.

Suggestions include being named after the parish of St Nicholas in Harpenden, through which it runs; Hemel's connection with Nicholas Breakspear; the knickerbockers worn by the navvies who constructed the line; or "down the nick", a slang term of engine drivers which meant "to run out of steam" and may have been applicable on the line's difficult inclines.

[2] The L&BR's construction had been delayed for several years by vigorous lobbying by a number of powerful and well-connected local landowners, including the eminent surgeon Sir Astley Cooper of Gadebridge House, who were all keen to protect their estates from invasion by the "iron horse".

[4] However, Grover's design found a sponsor and, following an act of Parliament in 1863, the Hemel Hempsted[notes 3] and London & North Western Railway Company (HH&L&NWR) was formed to construct and operate the line.

However, no construction work was undertaken due to difficulties with local landowners and problems agreeing the connection to the main line at Boxmoor; after a number of years of stagnation the earlier proposals were re-examined.

By 1865, the Midland Railway was developing its route out of London St Pancras, opening up new interchange possibilities to the north of Hemel Hempstead.

Eventually, the HH&L&NWR company ran into financial difficulties and it was the Midland Railway that came to the rescue, financing completion of the line and agreeing to operate it once it was built.

[6] The line was finally opened on 16 July 1877 to great fanfare with celebrations led by the Berkhamsted Rifle Corps Band.

The LNWR regarded the Hemel Hemptead and Midland Railway route to St Pancras as a threat to their service into Euston.

[8] The LNWR began to operate a regular horse bus service to transport passengers from the town centre to the main line station at Boxmoor, where they could take trains to London Euston.

[12] Eventually, the straw plait trade declined and the need for goods trains gave way to passenger demand; local businessmen wanted a fast route into London without having to go via Luton.

[12] The LMS also inherited the bus service which the LNWR had been running from into Hemel town centre; rather than transfer passenger traffic to the Nickey line, the LMS decided to extend the bus service to run from Boxmoor to Harpenden instead, duplicating the railway route which it then reduced to two trains per day.

[12] In 1930, the LMS experimented with new transport technology in an attempt to reduce costs and to rationalise its bus and rail operation through Hemel Hempstead.

The Nickey line was used to trial a hybrid road–rail vehicle system called the "Ro-Railer", a bus that could travel on both roads and railways.

[17] During the harsh winter of 1946–47, a national coal shortage hit the British economy[18] and passenger trains were "temporarily" suspended.

The line between Cupid Green (north of Godwin's Halt) and Harpenden survived for some more years, however, having been leased by British Rail in 1968 to the Hemelite concrete company who continued to use the line privately to transport raw materials for manufacturing building blocks via Harpenden to their works at Claydale.

The last journey on the line was made by a Hemelite Drewry locomotive, which was taken off the tracks at Redbourn and transported by lorry to work on the Yorkshire Dales Railway.

A combined iron drinking fountain and gas lamp still stands near the site of the now demolished Heath Park Hotel, which had stood directly opposite the railway station.

The course of the connection from Boxmoor is discernible in places; the gasholders at Duckhall are still present and the boundaries delineate the curve of the original trackbed.

The abutment of the bridge that crossed the A41 is still present on the northern side, followed by a few hundred yards of heavily wooded embankment which still have one or two remaining sleepers.

From Heath Park, the line went roughly to the site of the present day "Magic Roundabout", where it crossed the lower end of Marlowes over a viaduct.

The line can be traced easily across Keen's Fields, to cross Queensway on a high-level brick arch bridge which is still complete.