Originally a portion of the Taff Vale Railway's main line to Cardiff's Bute Docks, in 1922, it was absorbed, along with the neighbouring Rhymney Railway, into the enlarged Great Western Railway. With the decline of coal traffic and the closure of the Bute Docks, it now sees only passenger services, and is regarded as a branch from the line through to. At privatisation in 1995, services were operated by the Cardiff Railway Company, which traded as Valley Lines. This was subsumed by the new Wales & Borders franchise in 2001, which was subsequently awarded to Arriva UK Trains in December 2003 and operated as Arriva Trains Wales. In October 2018, Transport for Wales took over the franchise from Arriva Trains Wales. The December 2005 timetable introduced a further increase in services to 4 trains per hour 18 hours a day, and even a Sunday service for the first time. In December 2005, Arriva employed a single car Class 153 to "shuttle" along the Butetown Line, upgrading from the 2 car Class 143 'Pacers' used for the service. Since then, the service frequency has been increased even more – there are now 5 trains per hour on the line every day of the week, which equates to one train every 12 minutes. In July 2006 the service was due to be provided by a 1950s "Bubble car" DMU. The unit finally entered service on 17 August 2006, only to be withdrawn for repairs two days later. The unit then re-entered service on 14 September 2006.
Passenger volume
Below is the passenger volume from the year beginning April 2002 to the year beginning April 2015. Comparing the two years, Cardiff Queen Street's usage has increased by 44% whilst Cardiff Bay has increased 569%.
Modernisation
On 16 July 2012, plans to electrify the line were announced by the government, as part of a £9.4bn package of investment of the railways in England and Wales. The announcement was made as an extension of the electrification of the South Wales Main Line from Cardiff to Swansea and the electrification of the south Wales Valley Lines at a total cost of £350 million. It was proposed to start between 2014 and 2019. In June 2018 it was announced by the new operator Transport for Wales Rail that the line was to be re-integrated into Valley Lines services, with 6 trains per hour to operate from Merthyr Tydfil, Aberdare and Treherbert. New stations will be constructed at Loudoun Square and on a short extension from the current station closer to Cardiff Bay, opening in December 2023. The current Cardiff Bay station will close at the same time. Stadler Citylink tram-trains will replace the Class 153s. These will switch to battery power on the branch, negating the need for electrification.
