Question: Who operates the services on the line where Watford Junction Motive Power Depot was situated?

Context paragraphs:
Motive power: In thermodynamics, motive power is a natural agent, such as water or steam, wind or electricity, used to impart motion to machinery such as an engine.  Motive power may also be a locomotive or a motor, which provides motive power to a system.  "Motive power" may be thought of as a synonym for either "work", i.e. force times distance, or "power".

Camden motive power depot: Camden Motive Power Depot was a railway motive power depot, close to Chalk Farm, Camden in London, England from 1837 until 1966, servicing express passenger locomotives using Euston Railway Station.  It was closed following the electrification of the West Coast Main Line and largely demolished.  However, part of the original facility has been preserved as The Roundhouse centre for the performing arts.

Northwich Motive Power Depot: Northwich Motive Power Depot was a Traction Maintenance Depot located in Northwich, Cheshire, England.  The depot was situated on the Mid-Cheshire line and was located immediately to the southeast of Northwich station.

Bahnbetriebswerk: A Bahnbetriebswerk is the equivalent of a locomotive depot (or motive power depot) on the German and Austrian railways.  It is an installation that carries out the maintenance, minor repairs, refuelling and cleaning of locomotives and other motive power.  In addition it organises the deployment of locomotives and crews.  In the Deutsche Bahn, a "Bahnbetriebswerk" is known today as a "Betriebshof"; the ÖBB refer to it as a "Zugförderungsstelle" ("Zf").  Many other countries simply use the term 'depot'.  The smaller facility, the "Lokomotivstation" (also "Einsatzstelle" or "Lokbahnhof") akin to the British sub-depot or stabling point, is affiliated to a "Bahnbetriebswerk".

Croxley Rail Link: The Croxley Rail Link is a railway engineering project in the Watford and Three Rivers districts of Hertfordshire, England.  The result will connect the two urban lines serving Watford, the London Overground and the London Underground Metropolitan line, at Watford Junction , improving interchange between lines.  The Metropolitan line Watford tube terminal station will close, and the line will be diverted and extended from Croxley tube station to Watford Junction via a reopened section of closed line.  Two new stations will be built on the reopened line.  The main proponent of the scheme is Hertfordshire County Council, with the support of Transport for London (TfL) which owns the Watford branch.

Watford DC line: The Watford DC line is a commuter railway line from London Euston to Watford Junction.  Services on the line are operated by London Overground.

Traditionsbetriebswerk Stassfurt: The Traditionsbetriebswerk Stassfurt (Stassfurt Heritage Locomotive Depot) is a German railway motive power depot at Stassfurt in the state of Saxony-Anhalt that was opened in 1856.  Today the locomotive depot, that lies on the Schönebeck to Güsten railway line, is operated by the society "Eisenbahnfreunde Traditionsbahnbetriebswerk Staßfurt" (Railway Friends of the Stassfurt Heritage Locomotive Depot).

Watford Junction Motive Power Depot: Watford Junction Motive Power Depot was a Traction Maintenance Depot located in Watford, Hertfordshire, England.  The depot was situated on the Watford DC line and was near Watford Junction station.

Radyr Motive Power Depot: Radyr Motive Power Depot was a Traction Maintenance Depot located in Radyr, Cardiff, Wales.  The depot was situated on the Merthyr Line and was near Radyr station.

Tinsley Motive Power Depot: Tinsley Motive Power Depot, latterly Tinsley Traction Maintenance Depot (TMD), was a railway depot in Tinsley, South Yorkshire, near Sheffield.  Access by road was from Brinsworth, near Rotherham.  The depot was situated on the freight line between Treeton Junction and the A631 Shepcote Lane.

Supporting paragraphs: Watford Junction Motive Power Depot Watford DC line
Answer: London Overground