A passenger on a BTS skytrain posted a video on online of water falling steadily from the roof of an airconditioned carriage, as commuters scrambled to stay dry.
Facebook user Supanut Siriudomdejkul posted the short clip on Wednesday night, showing water pouring from the roof where two carriages were joined. It also showed a large puddle of water slopping from side to side on the floor, washing against commuters' shoes in time to the swaying of the moving carriage.
"The first-hand experience is shocking. There was a sudden downpour, followed by drips with the rhythm of the turns," Mr Supanut wrote. His message was followed by an ongoing BTS-related hashtag in Thai meaning, "What's wrong with BTS today?"
Problems have plagued the country's first electric railway system recently. In late June, signal frequency interference caused serious delays during rush hours, affecting about 400,000 commuters.
Early this month, a carriage door opened while a train was running between stations, prompting the operator to repeat its warning for passengers not to lean on doors.
Last week, a woman fainted and fell from a station platform onto the tracks, triggering renewed calls for safety barriers.
Bangkok Mass Transit System Plc (BTSC), the operator, later stated that the water flowed from an air return vent of the carriage No. 3233 of the train No.33 while it was running on the Silom Line from the National Stadium station to the Bang Wa station on Wednesday evening.
It was the fourth carriage added to the train six years ago and Siemens was responsible for its maintenance. The carriage was later removed from the BTS system for thorough examination and repair.
The examination found that the air-conditioner of the carriage did not drain water and it was replaced. Siemens was checking the removed air-conditioner and would also examine the other air-conditioners of the same make to prevent the incident from recurring.
BTSC apologised for the incident.