In 1872 the Cape Government, under Prime Minister John Molteno, ordered that a railway line must be constructed across this barrier in the vicinity of the Hex River Mountains.

In 1874 surveyor Wells Hood, under the instruction of Brounger, found a potential route up the 2,353 feet (717 metres) climb from De Doorns in the Hex River valley to the top of the Karoo plateau east of the Valley, that would require gradients of no more than 1 in 40 uncompensated, very steep by railway standards, and tight curves with a minimum radius of 100 metres (328 feet).

[1][2][6][7] By 1876, the Molteno Government had selected Thomas Brounger's proposed route through the Hex River valley, with the line to follow the route from Worcester through De Doorns, then along Hood's proposed pass across the mountain via Osplaas to the 3,147 feet (959 metres) summit at Matroosberg, and then via Kleinstraat to Touws River.

Credit for the fact that most of the present day railway lines in Africa are Cape gauge can therefore be directly attributed to the Hex River rail pass.

On the route ascending the mountain, Osplaas provided the only level stretch that was long enough for a conventional passing loop.

[9] This first tunnel served the railways for 53 years, until the track was re-laid in 1929 to diminish a curve to accommodate larger locomotives.

[7][8][10] Despite its quick and relatively cheap construction, the Hex River rail pass served the South African Railways (SAR) for more than a century.

It was the starting point of the country's first railway line to the Witwatersrand and opened the way for Cecil Rhodes' colonisation thrust into central Southern Africa.

[1] This eventually led to the decision to construct a tunnel system to eliminate the Hex River rail pass altogether.

Evans, later to become Chief Civil Engineer of the SAR, proposed a new route for the section between De Doorns and Matroosberg that would result in a gradient of 1 in 66 compensated and a minimum curve radius of 800 metres (2,625 feet).

The length of the section would be reduced by 8 kilometres (5 miles) and it would also eliminate altogether 5,280 degrees of curvature and 110 metres (361 feet) of false rise in level.

The original eastern portal (coordinates 33°24′21″S 19°54′03″E / 33.405814°S 19.9008°E / -33.405814; 19.9008) was dug immediately adjacent to the N1 national road some 15 kilometres (9 miles) west of Touws River and took the form of a cutting into a gradient to sufficient depth to commence tunnelling.

Work was eventually resumed in 1974 and included the remodelling of the lower section of the deviation between De Doorns and Osplaas stations as well as the construction of tunnel no.

After the engineering, geological and economic factors had been analysed, the straight route with a horseshoe profile and concrete lining was finally selected.

[1] The tunnel was constructed by Compagnie Interafricaine De Travaux (Comiat), a division of Spie-Batignolles in Paris, France.

The contract called for both to be widened to new design standards to allow for overhead electrification and broader loading gauge clearances.

The present longest single railway tunnel in Africa is 15.5 kilometres (9.63 miles) long, on the Gautrain line between Johannesburg Park Station and Marlboro Portal, which was broken through in September 2009.

A large part of the track between Matroosberg and Osplaas stations was left in place, however, and is now being utilised as a popular tourist destination known as the Hexpas Ecotrek.