A mineral branch from Pontypridd to Dinas Rhondda opened in June 1841. The Llancaiach Branch was authorised in the original Act for the TVR. It opened on 25 November 1841 for mineral traffic only, from Stormstown south of Abercynon to three adjacent collieries at Llancaiach. There was a self-acting rope-worked incline long on a 1 in 8 gradient. Use of the line was less than expected, traders finding that the charges on the canal were substantially lower. The TVR hesitated to build the authorised branch to the tramroad to Dowlais and the clause in the Act enabled the Dowlais Iron Company to take over the construction and the branch, which they did. The line was open between Merthyr Tydfil and Cardiff, but almost immediately thought was given to improving the capacity of the main line and of serving the coal production of adjacent valleys. The line was originally single, except for the Quaker's Yard and Llancaiach inclines, which were double. Cardiff to was doubled in 1846 and through to Navigation House (Abercynon) in 1847. The doubling was completed to Merthyr in 1862. The original purpose of the TVR had been deeply connected with the ironworks of Merthyr. In the years immediately following the railway's authorisation, the rich seams of high-quality coal in the Aberdare area came into prominence and began to outshine the Merthyr trade. Sixteen steam coalpits were sunk there between 1840 and 1853. Those collieries required transport to the sea and the proprietors of the TVR responded by sponsoring the Aberdare Railway, a nominally independent company. The Aberdare Railway was incorporated on 31 July 1845 to make a branch from Navigation House to . It was worked by the TVR from the outset and leased to it from 1 January 1847. It opened for passenger and goods and mineral traffic on 6 August 1846. Navigation House station was renamed Aberdare Junction on the opening of the branch. There was a branch off the Aberdare line from Cwmbach to Abernant colliery, also opened in 1846; it crossed the River Cynon to get access. In 1847 the Newport, Abergavenny and Hereford Railway obtained powers to build its Taff Vale Extension westwards from Pontypool to connect the TVR at Quakers Yard. This would be the first connection with the rest of the standard-gauge railway network; the TVR reviewed its policy of right-hand running and decided to change to left-hand running in preparation for the connection.