The western portal (coordinates 33.415182°S 19.765646°E) of tunnel no.
4, as established in 1948, enters directly into the mountain face, which is nearly vertical at that point.
The eastern portal (coordinates 33.40843°S 19.908717°E) was relocated a short distance to the southeast of the original 1948 portal and is in a 600 metres (1,969 feet) long and 16 metres (52 feet) deep cutting.
The tunnel is 13.5 kilometres (8.4 miles) long and has a maximum cover of 250 metres (820 feet).
The gradient is mainly 1 in 66, except at the passing loop where it decreases to 1 in 200.
Five ventilation shafts of 1.8 metres (5 feet 11 inches) diameter and with a combined length of 1,000 metres (3,281 feet) were sunk.
The cross-sectional area of the horseshoe profile single line tunnel is 30 square metres (323 square feet), but this increases to 66 square metres (710 square feet) at the passing loop.
Tunnel no.
4 also contains relay rooms for signalling equipment.

The tunnel system became operational in April 1989, more than forty years after the first portals were sunk, and was officially opened on 27 November 1989.
The completed four-tunnel system now boasts the longest railway tunnel system in Africa.

Tunnel 4 was the longest railway tunnel in Africa until 2009.
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.
The western portal (coordinates 33.415182°S 19.765646°E) of tunnel no. 4, as established in 1948, enters directly into the mountain face, which is nearly vertical at that point. The eastern portal (coordinates 33.40843°S 19.908717°E) was relocated a short distance to the southeast of the original 1948 portal and is in a 600 metres (1,969 feet) long and 16 metres (52 feet) deep cutting. The tunnel is 13.5 kilometres (8.4 miles) long and has a maximum cover of 250 metres (820 feet). The gradient is mainly 1 in 66, except at the passing loop where it decreases to 1 in 200. Five ventilation shafts of 1.8 metres (5 feet 11 inches) diameter and with a combined length of 1,000 metres (3,281 feet) were sunk. The cross-sectional area of the horseshoe profile single line tunnel is 30 square metres (323 square feet), but this increases to 66 square metres (710 square feet) at the passing loop. Tunnel no. 4 also contains relay rooms for signalling equipment.
The tunnel system became operational in April 1989, more than forty years after the first portals were sunk, and was officially opened on 27 November 1989. The completed four-tunnel system now boasts the longest railway tunnel system in Africa.
Tunnel 4 was the longest railway tunnel in Africa until 2009. 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.
Hex River Tunnels