INVESTIGATORS were last night trying to find why a Mass Transit Railway train broke down during the rush hour, leaving thousands of commuters stranded. Passengers waited up to 50 minutes for trains, while some took advantage of extra bus services and two extra ferry services. Delays began at 8.30 am after a set of doors on a Central-bound train would not close at Kwai Hing station on the Tsuen Wan line. rther delays were caused when the train broke down after leaving Kwai Hing station. Engineers took about half an hour to fix the train before they could move it to Tsuen Wan depot for inspection. Thousands more commuters, unaware of the problem, tried to enter Kwai Hing station and others on the line. MTR external affairs manager Maggie So said: ''The train was broken down and blocked the railway track when it was driven to its Tsuen Wan plant for inspection, so services between Lai King and Tsuen Wan were suspended.'' Services between Tsuen Wan and Kwai Fong stations were disrupted for 50 minutes, and trains between Lai King and Central stations could not be maintained at the usual 113-second intervals. Long queues resulted at the Tsuen Wan MTR bus terminal. Two extra buses and two ferries were provided between Tsuen Wan and Central. Full services on the Tsuen Wan line were not restored until 9.20 am. Ms So said announcements of the breakdown were made at all stations and on radio and television. She said passengers were given a refund and stored value ticket holders had a free ride. ''Passengers with single-journey tickets whose trips were disrupted can change their tickets within three days,'' she said. The result of the investigation is expected to be released soon, but the MTR declined to specify a date. An estimated 2,500 passengers were asked to alight and the train was sent back to the terminus at Tsuen Wan for repairs. But fu