Initial developments included two two-road engine sheds, built east of the station (1866) and north of the station. In 1899 a substantial eight-road engine shed was built east of the station.
A new train-wash and additional maintenance building for Class 700 units has been constructed on the site of the old Coronation sidings, together with an underframe cleaning building alongside the current shed.
1850 & 1866 GNR engine sheds
A two-road dead-ended shed was established by the Great Northern Railway (GNR) in 1850, on the east side of the station. The shed closed in 1866 when the nearby Wood Green shed had been built, and was later demolished; making way for expansion of Hornsey station.[map 1]
In 1866 another two-road dead-ended shed was established ("Wood Green shed"), ~1 km north of the station on the west side of the track adjacent to a new water works, and west of Wood Green common. The shed closed in 1899.[map 2]
1899 GNR eight-road engine shed
In 1899 an eight-track shed was constructed to the east of Hornsey station, together with a 52 ft (16 m) turntable, coal stage and water tank; the shed was connected via the Ferme Park sidings.[map 3] The shed provided locos for shunting in the yard and nearby Ferme Park sidings, as well as goods workings across London to the south via Snow Hill tunnel. Hornsey locos shared suburban duties over the southern end of the GNR with locos from Kings Cross Top Shed.
With the passing of steam, in 1961 the shed was converted for use with diesel traction, and continued in use until the early 1970s, closing in May 1971. The shed's duties were passed to nearby Finsbury Park diesel depot.
1973 Great Northern electrification EMU depot
In the 1970s the site was extensively rebuilt; part of the Ferme Park sidings south of the shed was cleared, and a new Electric multiple unit depot constructed;[map 4] the work was done as part of the Great Northern electrification; a new maintenance shed was constructed with six roads, each capable of holding an eight-car train. At the same time the old loco shed was converted to use as an Overhead Line (OHL) maintenance depot, stabling an OHL repair train. 
After 1973 the depot code became HE.
In 2008 First Capital Connect opened a 'Driver Training Academy' at the depot in 2008, equipped with British Rail Class 319 and Class 365 driving simulators. The simulators became operational in 2009.
In 2008, the Department for Transport commissioned a study into the location of depots for the future Thameslink rolling stock: Network Rail preferred two depots based on an expectation that at times the central area of the Thameslink route would be closed for maintenance outside commercial operational hours, with no workable alternative electrified routes available. A single-depot solution was also investigated, but no suitably large sites were identified for such a facility. Sites were considered at: Wellingborough;[note 2] Hornsey;[note 3] Cricklewood;[note 4] Selhurst;[note 5] Three Bridges;[note 6] and Tonbridge. By late 2008, the sites had been narrowed to Hornsey, Three Bridges and Tonbridge; finally Hornsey and Three Bridges were selected as a two-depot solution.
Network Rail submitted a planning application in August 2009.
The development was split across sites east of the main railway line: the main maintenance building was north of Hornsey railway station and the A504 road (High Street/Turnpike Lane); storage sidings were located south-east of Hornsey station, adjacent to the pre-existing depot. The northern site was on rail sidings ('Coronation sidings'/'Hornsey sidings') which had been developed on made embankments from the later 19th century to early 20th century; the southern stabling area was on land that had been extensively developed as railway sidings since the early 20th century. The proposed development required bridge widening of the crossing at Turnpike lane, and additional embankment work and extensions.
The main maintenance building was a six-road 40 by 280 m (130 by 920 ft), 13.4 m (44 ft) high single-ended train shed, with a train wash to the west and a two-storey 12 by 117 m (39 by 384 ft) warehouse adjacent to the east. The southern site included sidings for 11 twelve-car trains and 2 eight-car trains, with cleaning and controlled emission toilet facilities.
In 2009, John Denham, Secretary of State for Communities and Local Government blocked the development of the depot – the development had been subject to local and council objections on grounds including negative impact to a conservation area, as well as a potential negative impact to Haringey Council's "Haringey Heartlands" redevelopment project.
A revised two-depot plan was produced: the southern (Three Bridges) depot was expanded to a five-road shed, while estimates for total maintenance roads required had been reduced from nine to eight. The northern depot was required to be only a three-road depot; the depot was to be suitable for twelve-car trains, with wheel lathe, cleaning, warehousing and stabling facilities. Potential sites for the northern depot were reassessed and possible options reduced to three: a main depot at Coronation Sidings Hornsey; a main depot adjacent to the existing depot at Hornsey; and a site at Chesterton, Cambridge – a depot reduced in size on the site of the original plan was chosen as the best option for Network Rail.
The revised plan was submitted in 2011, with the main depot reduced approximately 2 m (6 ft 7 in) in height, and nearly half the area, and with estimated employment figures reduced from 270 to 120 people. Rail union RMT criticised the loss of employment opportunities due to the reduced scale of the plans, stating "Without anyone trying, Haringey has lost 150 jobs"; the revised scheme continued to produce significant opposition from local residents, with concerns including noise and light pollution during night working, as well as visual impact and traffic.
The main building was a 278 by 21.6 m (912 by 71 ft) three-road 11.3 m (37 ft) high single-ended shed, with a two-storey 182 by 12 m (597 by 39 ft) warehouse adjacent east, and a 50.6 by 6.5 m (166 by 21 ft) train-washing building adjacent west; facilities at the depot were to include equipment for heavy overhaul, with overhead synchronised lift cranes on one road. The southern site, adjacent to the pre-existing First Capital Connect (FCC) EMU depot, would include CET facilities, another 50.6 by 6.5 m (166 by 21 ft) train-washing building, a 260 by 7 m (853 by 23 ft) underfloor cleaning building, and shunter's cabins. Wheel-lathe facilities were to be shared with a pre-existing installation at the FCC depot. Existing sidings adjacent to the FCC depot were to be rebuilt to provide 16 roads – four arrival and departure roads, and 12 storage roads. Civil engineering work included bridge-widening across Turnpike Lane (A51) and a culverted waterway, as well as minor embankment works, and rebuilding of a footbridge at Hornsey station.
The plan was given permission in late 2011.
The depot officially opened on 13 December 2016.
Hornsey depot is currently an Electric Multiple Unit depot  for Class 365, Class 387, Class 700 and Class 717 units. These units are used on the Thameslink and Great Northern Routes, Thameslink units are brought to Hornsey for maintenance tasks such as wheel turning, which Bedford Cauldwell depot is unable to do.
Facilities include a wheel lathe, large maintenance shed with lifting facilities and a train-washing plant, and extensive stabling sidings for Great Northern's commuter stock.
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A new maintenance depot to be built at Hornsey will include an inspection shed to accommodate 48 coaches on six lines. All 90 electric multiple-units will be maintained here. Five of the six eight coach long lines [..] will be used for light repairs and interior cleaning [...] The sixth line will have jacking equipment for carrying out heavy repairs [...] The site of the former Hornsey engine depot will be adapted as an overhead line equipment maintenance depot
- Ordnance Survey. 1:10560; 1967-8. 1:10000; 1973-6, 1981-4
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- Variously referred to as Hornsey E.M.U.D. (Hornsey Electric Multiple Unit Depot), Hornsey T&TRSMD, Hornsey EMU Depot, Hornsey Light Maintenance Depot, Hornsey LMD, or just Hornsey depot.
- Including sidings used by GB Railfreight.
- Adjacent to the existing Hornsey EMU depot then operated by First Capital Connect.
- On development land associated with the planned Brent Cross Thameslink railway station.
- On the site of the existing Selhurst Depot used by Southern.
- A split site on either side of the Brighton main line.
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