Rheneas Tunnel was built in 1865 and, along with Rheneas Viaduct, was the major engineering feature of the line. This three mile section is the one most subject to the weather's vagaries and is very sinuous. The tunnel is short, but curved, so that on entering, it is impossible to see right through and thus have warning of any obstruction. It is also unlined and subject to seepage. This sometimes loosens rock fragments; icicles also form here in frosty weather.
North of the tunnel the line runs along a ledge, and is subject to "wash-outs" after heavy rain. Regular inspection is therefore necessary, and inevitably under the worst weather conditions. This has resulted in the section being generally known to SR gangers as "that dratted tunnel section". A "wash-out" occurred here in 1867, which resulted in Skarloey getting stuck in a landslide. Another wash-out occurred in 1961, destroying a section of the line. A week later, Peter Sam lost his funnel to an icicle that had formed in the tunnel. Another notable incident involving the tunnel occurred in 1958, when a lose rock fragment halted Duncan. Incidents like these are typical of many others.