The Pontypool, Caerleon and Newport Railway was promoted independently to relieve congestion on the heavily worked Eastern Valley Line of the Monmouthshire Railway and Canal Company. The Great Western Railway put up half the capital, making it in effect a GWR subsidiary. It opened in 1874, and most long-distance passenger and goods traffic, especially the heavy mineral traffic, transferred to it. It amalgamated with the GWR in 1876.
The Llantarnam Link, connecting the upper Eastern Valley network, was opened in 1878 and from that time most local traffic transferred to the line.
The main line was increasingly used for long-distance passenger and goods traffic, especially from Bristol and the West of England after the opening of the Severn Tunnel. As local traffics declined and were extinguished, the PC&NR main line remained a key part of the North and West Route from the Severn to Shrewsbury and from there to the Mersey, and North Wales, and carries that traffic at the present day.