A SENIOR Mass Transit Railway official has said a comprehensive review of the MTR safety system urged by legislators would not be necessary, even though drivers would apparently not be alerted if carriage couplings came loose. An inquiry into Tuesday's MTR incident is aimed at finding out whether any warning signals were given to the driver when a train split in two in the tunnel between Tai Wo Hau and Tsuen Wan. Operations director Mr Bill Donald said yesterday there were reports of passengers triggering the alarm to try to talk to the driver but the message was not understood. After the carriages separated, both sections of the train were brought to a halt by a central computer. Mr Donald said, under normal circumstances, the train operator would need instructions from the central control room to start a train on manual mode after an automatic brake. ''The train operator would not have started the train had he known of the couplings failure,'' he said. Mr Donald said Hongkong's underground rail boasted the lowest accident rate among rail systems in the world and said safety had always been an important concern for the company. He yesterday attended a special session of the Transport Panel, where independent legislator Mr Chim Pui-chung insisted that a comprehensive inquiry should be launched even though there were no casualties in this week's incident. ''The MTR should be accountable to the public,'' Mr Chim said. ''Whether it is due to human, or technical error, there should be an explanation.'' He said the investigation could also shed light on the coupling systems used in the Kowloon-Canton Railway and the Light Rail Transit. Members were also concerned that there was no communication link during the accident between the stations and passengers stuck in the tunnel. Mr Moses Cheng Mo-chi said there must be an improvement to communication devices. Mr Donald pledged to look into ways to improve communication. A five-member team led by the MTR's safety officer and including engineers from the British train and German couplings manufacturers, had been working round the clock on the inquiry, Mr Donald told legislators. He said the company team would assist the separate investigation ordered by Governor Mr Chris Patten and led by top British rail expert Mr Alan Cooksey. Government officials said Mr Cooksey, who was responsible for monitoring the Channel tunnel railway, was instructing local railway inspectors on preparatory tests before he could arrive on May 9. Mr Cooksey will examine the MTR's inquiry findings, due to be submitted to the Government by May 10, and compile his report with recommendations to the Governor. MTR investigators have removed the couplings from the train for examination to determine the cause of the accident. Three rear carriages on the eight-carriage train became detached in the incident. The train was taken from Tsuen Wan to the Kowloon Bay depot yesterday morning. The force of separation was evident from the torn metal at the section where the carriages separated.