A preliminary investigation into Tuesday's train crash in Johannesburg has indicated that an error during manual authorisation resulted in the collision at the Eloff extension in Selby, south of Johannesburg, that left 112 people injured.On Thursday the Railway Safety Regulator (RSR) said one train was en route from Faraday to Naledi station, while the other train was travelling from Booysens station destined for George Goch station. Both had been carrying passengers.READ: Prioritise signalling, portfolio committee says after train crashSeven commuters sustained serious injuries while 105 others had minor injuries."The preliminary findings indicate that the train control officer (TCO) issued manual authorisation to the train headed to Naledi at 06:54 to pass and proceed to Crown station. After a couple of minutes, the TCO authorised another train onto the same line," spokesperson Madelein Williams said in a statement."The TCO gave the latter authorisation without having first checked whether the line was clear. When interviewed, the TCO indicated that he based the authorisation on the assumption that the section would have been cleared by then. This led to the collision of the two trains at the diamond crossing, outside the Booysens station."Williams said there had been no supervision of train control officers at the signal cabin at the time of the incident.Investigation ongoing"The regulator has in the past instructed Prasa (Passenger Rail Agency of South Africa) to provide sufficient supervision to TCOs and to provide proper control and approval of manual authorisation. Accidents like these suggest that Prasa is in contravention of its own standard operating procedures as well as the instruction of the regulator," she said."The RSR, therefore, calls on Prasa to expedite the process of appointing section managers to enforce countersigning at CTC (centralised traffic control) as a matter of urgency in line with the special conditions of their permit."Williams said both crews took "substance abuse tests" before and after the crash. The results were negative."The RSR will continue its investigation in order to establish the contributing factors and the root cause of the collision," she added.