The 6.5-mile (10.5 km) route passes through town and countryside in the county of Hertfordshire, just outside the boundaries of the Oyster Card and London fare zones.

A sixth station was added at Garston in 1966 and a seventh at How Wood in 1988, to coincide with the electrification of the route at 25 kV AC overhead.

Mostly a local route, the construction of the Midland Main Line ensured that it never became popular with the critical market of passengers to London.

Never used by passenger services, Ordnance Survey maps indicate it closed between 1883 and 1898, although the embankment and an abutment facing Watling Street remain visible.

Passengers starting their journey at Watford Junction, which is staffed, must have a valid ticket prior to boarding the train, in compliance with National Rail conditions of carriage.

For several years, London Northwestern Railway ran services with a single Class 319 train, which was acquired in September 2015 by London Midland as part of a set of trains from Thameslink to replace Class 321 units (which had in turn been acquired by Abellio ScotRail to operate on Glasgow's suburban rail network[10]).

This is operated by a manual plunger for trains heading towards St Albans, and by a treadle towards Watford Junction.

Since 1995 the Abbey Flyer Users Group (ABFLY)[13] has been campaigning to secure the future of the line and encourage its growth.

[2][16] Longer-term proposals envisaged extensions into Watford town centre via Clarendon Road and High Street, and St Albans city centre,[17] possibly as far as St Albans City railway station, and possible re-instatement of the line to Hatfield.

Among the solutions evaluated by Hertfordshire are: a proposal to reopen the passing loop at Bricket Wood; running trains which skip certain stations; closing some stations; conversion of the line to light rail operation; and conversion of the line to a guided busway.

If approved, this scheme would have enabled an increase in train frequencies, as well as the introduction of a regular "clockface" timetable.