The fire in a West Rail train two months ago was 'a random event of an isolated nature' and proper safety measures have been taken since then, the Kowloon-Canton Railway Corporation said yesterday.
It said the fire in a transformer on the train's roof had been caused by a combination of a hidden insulation flaw and lightning that triggered a power surge.
The transformer on the southbound train exploded at about 9am on February 14, stranding about 1,000 passengers in the middle of the Tai Lam Tunnel.
The passengers were forced to walk 2km to safety. Eleven, including a pregnant woman, spent the night in hospital.
The corporation's general manager of rolling stock, Tony Lee Kar-yun, said the hidden imperfection, which occurred during manufacture, had not been spotted in tests by the factory.
'But this alone would not lead to the equipment failure,' Mr Lee said, blaming lightning for triggering the fault.
'It is worth noting that Hong Kong has experienced a particularly adverse weather period with more frequent lightning strikes in recent years,' he said.