Following the requisite Act of Parliament, the Rhymney Railway (RR) company was incorporated on 14^th July 1854 with the intention of transporting iron and coal from the Rhymney Valley to the docks at Cardiff and Newport. It was supported by the Trustees of the Bute Estate which had acquired the mineral rights in the Rhymney Valley and the first section of track opened in 1858 between Rhymney and Hengoed, where the intention was to link with the Hereford, Abergavenny, Newport line and Newport Docks. In fact, the next development was via Ystrad Mynach to the outskirts of Caerphilly and then down the escarpment to Walnut Tree Junction in Taffs Well where it linked with the Taff Vale Railway (TVR) that ran to Bute Dock East in Cardiff. A limited passenger service was also available.

Relationships with the TVR were not always good with disputes over running rights leading to delays. In July 1864 a new Rhymney Railway (Cardiff & Caerphilly) Act was passed allowing the Company to raise money to extend the line from Caerphilly to Cardiff. A second RR Act in 1867 allowed more money to be raised from investors. Work started on a tunnel from Wernddu, to the East of Caerphilly, through the mountain to Cefn Onn in 1866. 5 shafts were sunk and tunnelling progressed north and south from the foot of each shaft until they joined. The story that there is a kink in the line where 2 of the tunnels were not properly aligned is probably an urban myth as, apart from a curve at the southern end, the tunnel is remarkably straight. There was, however, difficulty with water coming in from the mountain above and 1,600 gallons a day needed to be pumped out of the tunnels. On completion the shafts became ventilation shafts for the railway line.

At the south end of the tunnel, Cefn Onn Halt stood 50 yards from its mouth. This was a request stop that mainly served picnickers in nearby Cefn Onn Park and the Lisvane Golf Course, but flagging down south bound trains could prove a risky business as the driver would often fail to spot passengers on the platform and speed

past on the way to Llanishen. There are several online anecdotes from people who missed work because of this after housing developments turned this quiet rural retreat into a commuter area. The station was closed in 1964 when it was replaced by Thornhill and Lisvane station to serve the needs of the nearby Thornhill estate. In the other direction, the station at Gilfach Bargoed has a short platform that can only be accessed from the front carriage of the train and passengers wishing to alight her must be careful where they sit.

The first train ran through the tunnel on 3^rd December 1870 with the first passenger service running on 1^st April 1871. Caerphilly station, originally 1 mile to the west at Beddau Halt (now Aber Junction) was opened on its current site the same year. With the opening of the Pontypridd, Caerphilly and Newport Railway (PC&NR) in 1884, Caerphilly Station became a 4 way junction between the north south running RR, PC&NR to the west and Brecon & Merthyr Railway to the east, which also gave access to stations to Newport and its vast Alexandra Docks. A branch line also existed from Aber Junction up the Aber Valley to Senghenydd supplying the Universal and Windsor collieries there as well as offering passenger services. Following the 1964 Beeching cuts to train services, this line now acts as a footpath and cycle track, while the Taff Trail runs over much of the PC & NR), although the RR line and the tunnel serve one of the main routes on the proposed South Wales Metro complex.

Originally, the main Engine Sheds for maintenance of trains and rolling stock were at Crockerbtown Bridge in Cardiff (the site of the current Queen Street Station). In 1868, Cornelius Lundy was tasked with purchasing land near the proposed RR route at the end of Van Road. He negotiated a favourable deal for 17 acres with the Plymouth Estate and, when the RR proved an enormous success, most of the work was relocated to the Caerphilly site where it remained until 1964. In the early 20^th century Caerphilly became known as “little Swindon” because of the volume of steam locomotive building and repair that took place there. At its peak it employed 600 men, but closed in 1963 when diesel had replaced steam. Cornelius Lundy himself became the first General Manager of Caerphilly Works as well as taking on additional roles such as Chief Engineer. He was finally asked to retire in 1905 at the age of 93! He was offered a directorship in compensation for his long service and surprised the Board by continuing to actively attend meetings until his death aged 95.

Today, although the occasional freight train still runs over the line at night, RR is almost exclusively devoted to passenger services and the number of trains between Cardiff and Caerphilly was increased from 4 to 5 per hour in 2024. Most services continue to Ystrad Mynach, Bargoed and Rhymney, although with decreasing frequency the further north you progress until only 2 trains and hour serve Rhymney during the day. In the last 2 years over half a million passengers a year have used the service, although this is lower than before the Covid-19 pandemic.

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Victorian enterprise and ingenuity builds a comprehensive train network connecting Caerphilly and the rest of South Wales, which is destroyed by Dr Beeching in 1964. Welsh Government is seeking to reopen the network on a smaller scale, with new Engine Sheds opening near the old Walnut Tree Junction in Taffs Well and is looking for funding from the UK Government to match that spent on High Speed Rail in England.