Conversion of Sudan Railways to diesel traction started in the late 1950s, but a few mainline steam locomotives continued in use in 1990, serving lines having lighter weight rails.[1] Through the 1960s, rail essentially had a monopoly on transportation of export and import trade, and operations were profitable. In the early 1970s, losses were experienced, and, although the addition of new diesel equipment in 1976 was followed by a return to profitability, another downturn had occurred by the end of the decade. The losses were attributed in part to inflationary factors, the lack of spare parts, and the continuation of certain lines characterized by only light traffic, but retained for economic development needs and for social reasons.[3] A number of South African diesel locomotives are in use in Sudan.