The truck carrying about 50 farm workers reportedly crossed onto the tracks and collided with a high-speed train.
The truck was split in two by the impact.
“There are pieces of bodies scattered over a two-kilometere area as the train dragged the truck in its path before it could come to a halt,” a paramedic said.
Thulani Sibuyi, head of Mpumalanga province's community safety department, confirmed that at least 30 people were killed in the accident while scores of others were being taken to various hospitals across the province.
Mpumalanga is South Africa's greenest province, with fruit farms scattered throughout the area.
The workers were apparently on their way to work to pick the citrus fruit which is now in season and for which the area is renowned.
The accident occurred on the main line linking South Africa with neighbouring Mozambique.
Paramedics on the scene described the train crash as one of the worst they have ever seen.