In 1973 and 1974, the South African Railways placed one hundred Class 6E1, Series 4 electric locomotives with a Bo-Bo wheel arrangement in mainline service.

A pantograph hook stick was stowed in a tube mounted below the bottom edge of the locomotive body on the roof access ladder side.

While some Class 6E1 series were visually indistinguishable from their predecessors or successors, some externally visible changes did occur over the years.

On the early Series 3 units in the number range from E1296 to E1345, an externally visible difference was a narrower stirrup below their side doors.

[3] During the 1970s, Dr. Herbert Scheffel of the SAR experimented with self-steering bogies which not only reduced flange wear on goods wagons, but also opened up the possibility of running at high speed in passenger service on Cape gauge.

E1525 reached a speed of 245 kilometres per hour (152 miles per hour) hauling a specially-adapted suburban coach on a stretch of track between Westonaria and Midway on 31 October 1978, a still unbeaten narrow gauge world speed record on 3 feet 6 inches (1,067 millimetres) Cape gauge.

Testing took place over a 10-kilometre long (6.2-mile) stretch of straight track between Rosslyn and De Wildt on the line between Pretoria and Brits.

E1525 is still dedicated to testing projects since its different gear ratio and traction effort curves make it unsuitable for use in multi-unit working with other Class 6E1 locomotives in the fleet.

This testing project eventually bore more fruit in 2011 upon the opening of the 1,435 millimetres (4 feet 8½ inches) standard gauge Gautrain which connects Pretoria, Johannesburg and the O.R.

1 end was stripped of all controls and the driver's front and side windows were blanked off to have a toilet installed, thereby forfeiting the unit's bi-directional ability.

The fact that the handbrake was located in cab 2 was not a deciding factor, but was considered an additional benefit.

In the 1990s many of the Series 4 units began to be repainted in the Spoornet orange livery with a yellow and blue chevron pattern on the cowcatchers.