The Kowloon-Canton Railway Corporation admitted yesterday it is stumped why doors on both sides of a train opened simultaneously last weekend. A company spokesman said the incident may have been caused by a faulty infra-red sensor which detects which side of the train the platform is on. The computer then allows doors to open only on that side. But engineers now believe the failure may have been caused by another fault they cannot identify. 'The cause was possibly a failure of the infra-red sensor coupled with another irregular situation whose cause cannot be ascertained at this stage,' the spokesman said. Legislators have pressed the company to take urgent action fearing passengers could fall out if doors are faulty. The problem train is still being examined by engineers and daily checks of sensors on all the company's trains have been stepped up, the spokesman said. Sensors are now being tested twice before each train goes into daily operation - usually only one check is carried out. 'We have already had a thorough check of sensors on the rest of the fleet and they are satisfactory,' the spokesman said. 'Examination of the train in question will continue until we find the fault.'