The Class 398 Citylink[7] is a fleet of 36 tram-train multiple units being built for Transport for Wales Rail by Swiss rolling stock manufacturer Stadler Rail.

The units will be equipped with traction battery packages, allowing them to operate over non-electrified sections of track in addition to those with 25 kV 50 Hz AC overhead supplies.

[8] The franchise award included a requirement that the operator perform a full replacement of its fleets;[9] as part of which process an order was placed with Stadler for delivery by 2023 of 36 tram-train units that would be capable of running on existing lines and also under battery power on the streets of Cardiff.

[11][12] The first unit to arrive in the UK was 398001, delivered from the Valencia works to the Rail Innovation & Development Centre in Melton, Leicestershire[13] for testing on the Old Dalby Test Track.

[1] When built, Transport for Wales Rail Class 398 units will operate services on the Rhondda, Merthyr and Cardiff City lines,[14] and the Cardiff Bay Branch.