Lôn Las Cymru
Coordinates: 53°18′32″N 4°37′59″W / 53.309°N 4.633°W
National Cycle Route 8 (or NCR 8) is a route of the National Cycle Network, running from Cardiff to Holyhead. The route passes through the heart of Wales, and is also known by its Welsh name Lôn Las Cymru (English: Wales' Blue Lane). It is largely north-south from Holyhead to Cardiff or Chepstow, and in total measures some 400 km (250 mi) in length. Some of its route follows the trackbed of former railway lines, such as Lôn Las Menai, Lôn Eifion, the Mawddach Trail and the Taff Trail; in other places, the route is on public highways. A road bike with racing tyres would struggle on the former surfaces; a hybrid or 'hard tail' mountain bike fitted with wide road tyres would be a better choice.
Both ends are easily accessible via the rail network. It is possibly better done North to South, finishing in Cardiff, the capital city.
This largely follows the Taff Trail:
Cardiff | Pontypridd | Abercynon | Merthyr Tydfil | Brecon
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Wikipedia