The Chester and Holyhead Railway was an early railway company conceived to improve transmission of government dispatches between London and Ireland, as well as ordinary railway objectives. Its construction was hugely expensive, chiefly due to the cost of building the Britannia Tubular Bridge over the Menai Strait. The company had relied on Government support in facilitating the ferry service, and this proved to be uncertain. The company opened its main line throughout in 1850. It relied on the co-operation of other railways to reach London, and in 1859 it was absorbed by the London and North Western Railway.

There were extensive mineral deposits at a number of locations south of the C&HR main line, and the C&HR and the LNWR encouraged the building of branch lines to serve them. Llandudno was an early centre of leisure and holiday travel, and in the last decades of the nineteenth century, that traffic became increasingly important. In the twentieth century, the North Wales coast became a popular holiday destination, reached largely by rail travel.

In 1970, the Britannia Tubular Bridge suffered a serious fire, and the line was closed at that point until 1972 when a new structure at the same site was brought into use.

The container traffic at Holyhead has ceased, and passenger connections to the Irish ferries are much reduced, but the entire original main line is still in use for passenger traffic, together with the Llandudno branch and the Conwy Valley line to Blaenau Ffestiniog.

In 1800 acts of the United Kingdom and Irish parliaments determined the union of the two countries; this took effect on 1 January 1801. Prior to that time, the countries had been in personal union, having a shared sovereign but independent governments.

There had long been a necessity for convenient communication between London and Dublin, and the union gave further emphasis to the need, specifically for government mails. The sea crossing of the Irish Sea was difficult because of the poor harbour facilities on both sides, and the road approach to the harbour at Holyhead, Wales was long and difficult. In 1815 Thomas Telford was commissioned to build an improved road between London and Holyhead, and also between Lancashire and Holyhead. The A5 road as it existed in the 1950s shares much of the heritage of Telford's London to Holyhead route. Telford's road was completed from Shrewsbury to Bangor in 1819, and across Anglesey in 1828, including the Stanley Embankment from Anglesey to Holy Island.

The London route avoided Conway, crossing the River Conwy at Betws-y-Coed, but the Lancashire route required to cross the river, and both routes needed to cross the Menai Strait. Telford built bridges for these crossings. The Conwy Bridge and the Menai Bridge were both completed in 1826. As part of the same project, Holyhead Harbour was substantially improved in 1821, and subsequently extended and further improved in 1831 and 1847.

Although other routes to Ireland were used (Liverpool in particular), Holyhead was now the obvious, and easiest, port of access. On the Irish side, Howth Harbour, north of Dublin had been used, but increasingly the small harbour at Dún Laoghaire south of the city was used; it had been renamed Kingstown in 1821 in honour of a visit by King George IV.