From RSSA Gauteng's On Track:
The first passenger haulage by the class 38 is believed to have been en route to Cape Town for three based there. After that they were used fairly frequently on trains to Magaliesburg and elsewhere. This was due to the vagaries of Spoornet's accounting system. Electric was cheapest, then diesel and steam was the most expensive. So the electric rate was paid from Johannesburg to Waterval and the diesel rate from thereon until the accounting people woke up and closed the loophole.
One public holiday, two class 38s with twelve coaches were booked to Magaliesburg. All went well on the outward journey but one failed before leaving Magaliesburg and the single loco hauled the whole consist back to Johannesburg. It has to be said that you could walk alongside the train on the steepest sections but it kept going. The reason, it is believed, is that the 38s had AC traction motors and thyristor controls. This allowed a lower factor of adhesion or a higher tractive effort for a given weight of locomotive than with DC traction motors.
Since basic physics says Power = Force x Speed if the Power is constant, the loco is able to supply a very high tractive effort at very low speed. In the USA this is seen in the Powder River Basin where the AC traction-motored diesel locos crawl out of the basin with enormous trains. They are the same power as the DC diesel locos but their lower allowable factor of adhesion due to AC traction allows them to pull substantially more but at a lower speed.
Trains Galore ran a unique haulage combination of class 25NC and class 38 on a three-coach corporate special to a dam near Pretoria. Both locos were under power and acceleration was startling. The steam loco was taken off at Hercules and the class 38 continued to the destination and back to Johannesburg. 'From memory the computer tripped out the system at 105km/h, which we attained on this trip and others. Most spectacular was doing 100km/h through Witpoortjie station and seeing the clouds of dust and rubbish kicked up in its wake.'
The 38s were not used only for shunting. They often ran with full loads on the main lines between Broodsnyersplaas and Welgedacht. A number of years ago, 38s were used to pull the "Witblitz" container express all the way from Johannesburg to Cape Town and back. This saved time by avoiding the need to change engines at Beaufort-West (AC to DC traction) and Kimberley (DC to AC traction). Some of the 120km/h speed limit boards were still in existence between Potchefstroom and Worcester up to 2005.
On Track credits Geoff Pethick, Willem de Beer, and Andre Kritzinger for the above