An investigation has been launched into what led to the collision of a passenger train and a rail maintenance locomotive which left several passengers injured in Mamelodi, Pretoria on Saturday.Gauteng Metrorail spokesperson, Lillian Mofokeng, said the accident happened at about 18:24 at Eerste Fabrieke Station.She said a "Transnet tamping machine busy with maintenance collided with the new Metrorail train... The tamping machine rolled from Greenview back to Eerste Fabrieke station where the Metrorail train was stationary and carrying commuters on-board".A tamping machine rolls on wheels like a train and is used to tamp the track ballast under the railway track. It belongs to Transnet.PICS: Newly launched train in collision in Mamelodi, 50 injured "As a result of the collision, the train service between Pienaarspoort and Pretoria has been affected. The line has been closed to allow both Metrorail and Transnet technical teams to attend to the accident," Mofokeng said.She explained further how the accident happened: "The driver of the stationary train requested the 300 commuters on board to leave the train upon noticing the oncoming tamping machine. The commuters refused to leave the train instead and pulled the passenger emergency alarm levers which stops the train from moving".Bestcare Ambulance Service spokesperson, Xander Loubser, was among the paramedics who responded to the scene.He said 50 patients were attended to. From that number, he said 39 sustained slight injuries while the other 11 were treated for serious injuries."There were no critical injuries or fatalities on the scene," Loubser said. Meanwhile, the accident will affect normal train operation on the affected route."Customers are advised to expect train delays on this corridor while Metrorail is addressing the problem. Metrorail would like to thank all its customers for their support, understanding during this difficult time," Mofokeng said.