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. Built to replace Class 313 trains on services into Moorgate, a total of 25 six-car units began entering regular service from March 2019.^[10][11] The units are stylistically similar to the Class 700s (in use with Thameslink) and the Class 707s (in use with Southeastern and South Western Railway.)

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.^[12] In December 2015, GTR announced that it had selected Siemens to provide this new fleet, originally designated Class 713,^[4] as a follow-on order from the main Class 700 order, with entry into service expected from March 2019.^[13] The order was finalised in February 2016.^[14][15]

A significant difference between Class 717s and the earlier Class 700s is the provision of fold-down emergency doors at both ends of each train. These are required for emergency evacuation of passengers^[9] while inside the deep-level Moorgate tunnels.^[16]

Siemens began testing the Class 717 units in Germany during June 2018.^[17]

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.^[21] Further testing and implementation on other lines and routes is planned.^[22]

In October 2022, testing started for the upgrade of the ETCS baseline from B3MR1 to B3R2, with dynamic testing planned for December 2023.

Class 717 trains generate electricity through regenerative braking.^[23] The trains are also 20% lighter than their predecessors, making them more energy efficient.^[24]