The Heart of Wales line (Welsh: Llinell Calon Cymru) is a railway line which connects Craven Arms in Shropshire to Llanelli in South West Wales. It serves a number of rural centres, including the 19th-century spa towns of Llandrindod Wells, Llangammarch Wells and Llanwrtyd Wells. At Builth Road, 2 miles (3.2 km) from the town of Builth Wells, the line crosses the former route of the earlier Mid Wales Railway, which closed in 1962.

Historically, the line was known as the Central Wales line (Welsh: Rheilffordd Canol Cymru) and also included routes through Gowerton, where the railway crossed the West Wales lines and ran through Dunvant and Killay then down through the Clyne Valley to Blackpill, and then along the sea wall to Swansea Bay, near to the former slip bridge, before finally reaching Swansea Victoria.

This section, built originally by the Llanelly Railway and Dock Company (LR) to compete with the Great Western Railway and break the monopoly they held on Swansea Dock, closed in 1964. Nationalisation of the railways had removed the need for competing routes, and the running down and closure of Swansea North Dock ended the need for freight services on this section. Trains now use the original LR main line to reach the West Wales lines at Llandeilo Junction and thence Llanelli and Swansea, after a reversal.

North of Llandovery, the route was opened in stages between 1861 and 1868 by a number of different companies, all backed by the London and North Western Railway (LNWR):

The Knighton Railway and the Central Wales Railway were given permission to amalgamate by the Central Wales Railway Act 1863 (26 & 27 Vict. c. lxxix).

The Knighton Railway Company, Central Wales Railway Company and the Central Wales Extension Railway Company were merged into the LNWR by the London and North Western Railway (Knighton, Central Wales and Central Wales Extension Railways Transfer) Act 1868 (31 & 32 Vict. c. xxxviii).

The 1963 Beeching Report proposed that the entire Central Wales line be closed but this was refused by the Ministry of Transport, except for the Pontarddulais to Swansea Victoria section. As a rural branch line, it survived the Beeching Axe since it carried freight traffic to the steelworks at Bynea and industrial areas such as Ammanford and Pontarddulais, linking them with the docks at Llanelli. It also passed through six marginal constituencies. During engineering work, the line is still occasionally used as a diversionary freight route. The basic service over the line has remained more or less constant since the 1970s, with four or five trains per day in each direction on weekdays, and two or three on Sundays.

The line is single track throughout, except for a few miles at the southern end shared with the Swansea District line, and has been operated under a light railway order since 1972. There are five passing loops, at Llandeilo, Llandovery, Llanwrtyd, Llandrindod and Knighton. Unless "Out of Course" working occurs, the Llanwrtyd passing loop is used on two of the Monday-Saturday services and the Llandrindod passing loop is used on the other two and also on the Sunday services. The signalling was modernised in 1986, when a system known as No Signalman Token Remote working was introduced. This is overseen by the signaller at Pantyffynnon, with the token instruments at the aforementioned five passing loops being operated by the train crew. The surviving signal boxes at each station were closed, as part of the modernisation scheme, and the points converted to automatic operation by British Rail.