The Government announced in January that it will install the doors first in Jurong East, Pasir Ris and Yishun stations next year, and the rest by 2012.
Transport Minister Raymond Lim had said that such incidents disrupted train services and inconvenienced many commuters, especially during peak hours.
It has not been decided how high these doors will be or what form they will take, but they will be of a 'sufficient' height and yet still allow for fresh air to circulate, said the Land Transport Authority (LTA).
Calls for installing such doors started about four years ago. The subject has also been mentioned in Parliament whenever someone strays onto the tracks.
The number of such incidents has been on the up: from an average of 16 cases a year in 2004 and 2005, to 30 in 2006 and 31 last year.
But each time the subject was raised, SMRT and the LTA raised the 'cost effectiveness' question, given the number of such incidents. The LTA also said the cost of the doors could eventually be borne by commuters in the form of higher fares.
But Mr Lim has explained that, with such screen doors being adopted in transit systems worldwide, their cost has come down.
LTA confirmed that the cost has fallen by a quarter, but declined to give specific numbers.