 The Class MF joined the experimental Class MD on the coal traffic line between Witbank and Germiston.They were acquired to improve the traffic flow on this line with its ruling gradient of one in one hundred (1%), where the increase of traffic and the resultant congestion was causing considerable delays en route which led to excessive hours of duty being imposed on crews.When the Mallets replaced the existing Class 11 locomotives on this line, train loads could be increased from to .