Water leakage was the cause of the platform screen door glitch which tripped up the Downtown Line on May 3.
Investigations by operator SBS Transit (SBST) found that water seepage had damaged electrical components of a screen door at the Botanic Gardens station.
The operator said yesterday that the leak was caused by natural groundwater seeping in from a space above the screen door - believed to be the tunnel ceiling.
The leak has since been filled with a polyurethane chemical to prevent water from getting through again, SBST said.
The platform screen doors at the station were last checked in September last year as part of routine maintenance. None was found to be faulty and no water seepage was observed then, SBST said.
But after this incident, all Downtown Line and North-East Line platform screen doors and their electrical components were inspected, and minor water seepage was found at two doors at the Downtown Line's King Albert Park station.
"However, their electrical components were not affected and we have since grouted the seepage," an SBST spokesman said. "We are working with the Land Transport Authority on the water seepage issue. We apologise to all affected commuters for the inconvenience."
Leaks in tunnels have been the root cause of several rail incidents, from short circuits that sparked tunnel fires to a major breakdown on the North-South and East-West lines on July 7, 2015.
Another major breakdown on the North-East Line in March 2012 was caused by corrosion of steel wires arising from tunnel water seepage.
Also in 2012, the Circle Line, then barely three years old, had to have all its power cables replaced because they had been compromised by water in the tunnels.
Dr Zhou Yi, council member of the Institution of Engineers, Singapore, said there are ways to make tunnels waterproof, but "there are cost issues".
"You may need to add another digit to the cost," he said. "But I think it's worthwhile to identify certain stretches of tunnels that need to be made waterproof. You don't need to do the whole line."
Singapore Institute of Technology Assistant Professor Andrew Ng said deploying "condition-monitoring devices to measure moisture content" would be a way to mitigate risk of water-related glitches.
The Botanic Gardens station door flaw affected at least 10,000 commuters from the start of service on May 3. Doors on the southbound side of the station could not operate automatically, and had to be operated manually. This slowed train service down and caused a crowd to build up on platforms.