Irish Rail is to run smaller Dart trains during quieter times of the day as it seeks to cut fuel bills by 3.2 million euro.
The company said it is to run two carriages at a time from mid-morning to mid-afternoon and after 7pm at night to cut out heavy energy use.
The off-peak initiative has been introduced on the back of research which shows almost two thirds of Dart passengers use the service in the four hours of rush hour in the morning and evening.
Only one third of its passengers on the Dublin suburban rail line are on the service on the other 14 hours.
Irish Rail said a range of energy saving initiatives already in place saved the company 16m euro last year alone and over the last five years overall energy consumption has fallen by a third.
Other schemes being introduced will target the intercity fleet, with 45 four carriage trains and 18 three carriage trains being created to allow managers to combine sets from three to nine carriage trains depending on demand.
"As well as savings in fuel and maintenance costs, the new fleet configuration will ensure train size can be targeted to meet demand, and increase train size and seat numbers on services which have experienced high numbers travelling," the company said.
The changes on the Dart will come into effect this week while changes to the intercity fleet will be phased in by the end of the year.
Irish Rail said that in total energy usage in megawatt hours has reduced from 793,400 in 2007 to 526,800 in 2012, a reduction of almost 34%.