Brunel's principal railway lines in England and Wales are listed at right. They include many branch lines and extensions completed after the construction dates shown (i.e., after his death).

Although South Wales was inaccessible directly by rail from southern England because of the width of the Severn Estuary — not bridged or tunnelled until a few decades after Brunel's death — he knew this barrier would be conquered, and had proposed a crossing himself in 1844. He saw the advantages to British industry of connecting the network to Welsh lines such as the Taff Vale Railway, which he designed. It connected the ironworks of Merthyr Tydfil, and collieries in the Cynon and Rhondda valleys, to Cardiff docks.

Many of Brunel's railways used timber for the construction of viaducts and bridges. The lines in Devon and Cornwall are usually associated with his timber spans. However, it was in South Wales that Brunel made the most extensive use of timber — not only for viaducts, bridges, piers and other infrastructure but also for buildings such as stations and goods sheds.

Brunel's first application of timber to railway works, on the construction of the Great Western Railway, were limited — a footbridge across Sonning Cutting (pictured) and a laminated timber viaduct he designed to cross the River Avon at Bath. However, he was already specifying the use of timber for bridges at Taffs Well and Pentyrch on the Taff Vale Railway, which was constructed at the same time. Timber would come to prominence in his work on the South Wales Railway, reflecting a need for economy in difficult times.

Brunel's largest timber works were constructed on the South Wales line and the Vale of Neath Railway. They include the longest-surviving completed timber structures, although none now remain. He erected a number of fan-style viaducts on the South Wales Railway, and the Dare Viaduct and Gamlyn Viaduct, on the Amman and Dare branches of the Vale of Neath Railway, are also of the fan type. These two viaducts were the last complete examples in active use and they never passed into British Railways' ownership.

Other principal structures included the Usk Viaduct at Newport, which burnt down in 1848 and was partially replaced, Landore Viaduct on the approach to Swansea, which was rebuilt in the 1880s, and Loughor Viaduct between Swansea and Llanelli. Until its demolition in 2013, the Loughor Viaduct was Brunel's last surviving example of a timber trestle viaduct in Wales — a reconfigured trestle substructure still supported on the original timber piles and carrying mainline rail traffic.

Before looking at the viaducts in more detail, plus timber viaducts by other engineers, let's pause for an outline of the career of Welsh engineer Rhys William Jones, who worked for Brunel on the survey for the South Wales line, and designed a timber trestle road bridge at Loughor that pre-dates the railway viaduct ...... next >