At least 20 DART trains were cancelled yesterday due to drivers reporting sick and refusing to work leave days.
Further cancellations are likely today and over the Christmas period.
DART services have suffered since Monday because of driver shortages, but the level of disruption was greater yesterday. Four DARTS were cancelled before 10 a.m., four more between 1 p.m. and 4 p.m., and 12 between 5 p.m. and 7 p.m.
It was uncertain last night what level of late service would be available.
Most trains were diverted southside and buses were provided for many northbound passengers.
The company's human resources manager, Mr John Keenan, said three out of six new DART drivers due to work yesterday had reported sick and a number of other drivers had refused to work rest days. This was in spite of an agreement with their unions that they would do so while extra drivers were trained.
Drivers recently accepted an agreement that provides £29,500 a year in return for a 48-hour week, of which 41 hours will actually be worked.
"It is very disappointing to find DART drivers are not honouring their agreements to provide a better service to customers despite the fact we have an agreement reached through the Labour Court," Mr Keenan said.
No union comment was available last night. The drivers are thought to be following the example of Irish Locomotive Drivers' Association members on the mainline services who are refusing to work rest days. It is unclear if the DART drivers, like the ILDA members, are seeking a review of recent pay deals.