The British Rail Class 717 Desiro City[9] is an electric multiple unit passenger train built by Siemens Mobility, currently operated by Govia Thameslink Railway on its Great Northern Hertford Loop and Welwyn stopping routes.

Upon winning the Thameslink, Southern and Great Northern franchise, Govia Thameslink Railway (GTR) announced that it would seek to procure new trains totalling 150 vehicles to replace the then-40-year-old Class 313 units operating on services to and from Moorgate.

[17] The first unit operated a single preview service in late September 2018, with gradual introduction from the spring of 2019.

[19] In September 2021, the installation of European Train Control System (ETCS) on the Class 717 fleet was successfully tested on the Thameslink 'core' route between St Pancras International and Blackfriars stations in central London.

[20] The success of the test allowed Govia Thameslink Railway to work towards enabling the use of ETCS in passenger service on the Northern City Line in November 2023, which will be followed by the decommissioning and removal of the existing conventional signalling system in 2024.