Three kms (2 miles) north of Kirkby Stephen station, the Settle and Carlisle line crosses Smardale Gill on a fine viaduct built from grey limestone. Along this steep and winding valley the Stainmore line made its way from Barnards Castle to Tebay, crossing in the valley the even more wondrous Smardale Gill Viaduct (see entry).

Smardale Viaduct is the highest on the Settle and Carlisle line at 39 m (130 ft). With 12 spans and a length of 218 m, it is the second longest (though only a little more than half the length of Ribblehad - see entry). It is a Scheduled Ancient Monument. The old North Eastern Railway line (formerly the South Durham and Lancashire Union railway) from Darlington to Tebay passed under this viaduct.

The Settle and Carlisle line of the Midland Railway is an outstanding piece of railway engineering. The engineer for the project was John Crossley, a Leicestershire man who was a veteran of other major Midland schemes. The terrain traversed is some of the bleakest and wildest in England, and construction was halted for months at a time due to frozen ground, snowdrifts and flooding of the works. One contractor had to give up as a result of underestimating the terrain and the weather - Dent Head has almost four times the rainfall of London.

The line was engineered to express standards throughout - local traffic was secondary and many stations were miles from the villages they purported to serve. It reaches a summit of 365 m (1,169 ft) at Ais Gill, north of Garsdale. To keep the gradients down to no steeper 1 in 100 (1%), a requirement for fast running using steam traction, huge engineering works were required but even so, the terrain imposed a 26 km (16 miles) climb from Settle to Blea Moor, almost all of it at 1 in 100, and known to enginemen as ‘˜the long drag.'.

14 tunnels and 22 viaducts were needed, the most notable being the 24 arch Ribblehead Viaduct. Soon after the crossing of the viaduct, the line enters Blea Moor tunnel, 2,404 m long and 152 m (500 ft) below the moor, before emerging again on to Dent Head viaduct. The summit at Ais Gill is still the highest point reached by main line trains in England.