Cardiff in making iron and later steel, brought to the sea by packhorse from Merthyr Tydfil. This was first achieved by the construction of a long canal from Merthyr (510 feet above sea-level) to the Taff Estuary at Cardiff. Eventually the Taff Vale Railway replaced the canal barges and massive marshalling yards sprang up as new docks were developed in Cardiff – all prompted by the soaring worldwide demand for coal from the South Wales valleys. At its peak, Cardiff's port area, known as Tiger Bay, became the busiest port in the world and – for some time – the world's
Cardiff Bay railway station building and its setting". In June 2018, new operator Transport for Wales Rail Services announced plans to build an extension and new terminus for the Butetown Branch, along with an intermediate station at Loudoun Square. These will replace the existing station, which will close. There is a shuttle service between Cardiff Queen Street and Cardiff Bay every 12 minutes Monday to Saturdays (between 0630 and 2330) and every 12 minutes on Sundays (between 1100 and 1630) using a Class 153 "Super Sprinter". The service was formerly operated by a Class 121 "bubble car", but this was withdrawn in June 2013.