This article does not cite any sources. (October 2016) (Learn how and when to remove this template message)
The Stadler Citylink (known as the Vossloh Citylink until 2015) is a series of tram-trains manufactured by Stadler Rail at its Valencia factory since 2003. They are currently used in Spain, Germany and the United Kingdom, with the Class 399 variant in service on the Sheffield Supertram network in the United Kingdom from September 2017 as the country's first tram-train. The second tram-train to be introduced in the UK will be introduced on the South Wales Metro in 2022.
In October 2011, Verkehrsbetriebe Karlsruhe ordered 25 Citylink tram-trains for operation on the Karlsruhe Stadtbahn network. The €75 million order also included options for a further 50 Citylinks. These Citylinks were known as the NET 2012, which stands for Niederflur Elektrotriebwagen 2012, German for "low-floor electric railcar".
The first Citylink was delivered to Karlsruhe in May 2014, around seven months behind schedule. Testing commenced on local railway lines on 19 June 2015. They are intended to replace high-floor trams on all lines except for Line 2, some of which are now up to 50 years old and are not wheelchair accessible.
Following their successful introduction, a further 25 NET 2012 vehicles were ordered in April 2015, scheduled for delivery from April 2016 onwards. An order for another 25 Citylinks was placed in March 2016.
Verkehrsverbund Mittelsachsen ordered eight NET 2012 tram-trains, with options for two more, for planned use on the Chemnitz Bahn network from 2015. In July 2015, a further four were ordered. Testing of the first tram-trains to be delivered commenced in September 2015. The Chemnitz Citylinks are fitted with a diesel generator to enable them to operate away from electrified routes, and they can also operate from 600 V DC and 750 V DC electrified lines. The centre section is also slightly higher, due to the differing platform heights in Chemnitz, allowing the doors in the centre section to be level with the higher platforms and the other doors to be level with the lower platforms.
The first Citylink entered service on the Chemnitz Bahn on 4 April 2016.
Nine Vossloh Citylink tram-trains were ordered by Ferrocarrils de la Generalitat Valenciana for use on the Alicante Tram network's lines 1 and 3. They have a maximum capacity of 303 passengers and a top speed of 100km/h, and are classified as the 4100 series on the Alicante Tram network.
Serveis Ferroviaris de Mallorca ordered six Citylinks in 2011 for operation on the new Manacor to Artá railway line, which was planned to reopen in 2012. However, construction of the line was suspended in 2013 and the Citylinks are now used on other lines across Mallorca.
Ferrocarrils de la Generalitat Valenciana ordered 62 Vossloh Citylink tram-trains for use on the Metrovalencia system's lines 1, 3 and 5, with deliveries taking place in 2007. They are classified on the network as Series 4300 units; 42 of them are four-car, with the remaining 20 units being formed of 5 carriages.
It was announced by the Department for Transport in 2008 that Sheffield would be used as the location for the first ever tram-train in the United Kingdom as part of a nationwide pilot scheme. Initially services were planned to be operated along the Penistone line from Sheffield to Huddersfield; this was later revised into a scheme operating along the Dearne Valley line between Sheffield and Rotherham Parkgate via Rotherham Central.
In 2013, Sheffield Supertram placed an order for seven Citylink tram-trains for use on the new Sheffield-Rotherham tram-train, that opened in October 2018. The Supertram Citylink design is based on that of the newer NET 2012 trams ordered by Karlsruhe and Chemnitz in Germany. Three tram-trains are required to work the service, with three being used to increase the number of services on the existing network and one in maintenance. The first tram-train was scheduled for delivery in September 2015, although this was later set back to December 2015.
Under Supertram's numbering system, the seven units were numbered as 201-207; however, because part of their operation is on the National Rail network, they are required to be registered under the TOPS used for main line rail vehicles, which led them to be allocated under TOPS as Class 399, numbered 399201-399207.
The Supertram Citylinks are dual voltage vehicles, capable of operating at the 750 V DC overhead electrification used on the Supertram network, and the 25kV AC overhead electrification being installed on the Dearne Valley line to Rotherham and used elsewhere on the National Rail system.
The first Supertram Citylink was delivered on 30 November 2015. The final vehicle was delivered on 20 November 2016. The first entered service on the existing network in September 2017.
South Wales Metro
In June 2018, it was announced that for the South Wales Metro, Transport for Wales Rail will purchase 36 3-car Stadler Citylink tram-trains. They will operate in train mode north of Cardiff Central whilst switching to battery tram mode on the Butetown branch line. These will serve the Merthyr and Rhondda lines as well as the Cardiff City Line. These vehicles are the first to feature "smart-electrification" which allows Discontinuous electrification with permanently earthed sections around restricted structures. These Tram-Trains will switch to battery mode under bridges and tunnels negating the costly intervention to over 50 structures on the South Wales Metro. 
- Rotherham tram-train replaces Penistone scheme Metro Report International 15 September 2009
- Sheffield tram-train enters service Metro Report International 15 September 2017
- Stadler is named preferred bidder to supply 71 trains for Wales & Borders Stadler Rail 13 August 2018