Iarnrod Eireann has said it may have to turn passengers away this Friday if large numbers try to take advantage of the CIE unions' decision to give 'free' fares for a day.
Workers in the CIE group are taking the action in protest at the government's decision to break up the company.
A spokesman for Iarnrod Eireann insisted that services would operate as normal on Friday.
However, he said the company would, if necessary, turn passengers away if capacity became a problem.
Asked whether the company was concerned that people might take advantage of the day to use Iarnrod Eireann services for free, the spokesman said:
"Obviously it's Friday and it's a busy day anyway, he said. We have to manage capacity and if large numbers of people show up and can't be accommodated, we will turn people away."
Bus Eireann said it had made no decision to cancel services on Friday. However, a spokesman said the company expected to place advertisements on Thursday to further inform the public.
A spokesperson for Dublin Bus could not be contacted for comment on the company's plans for handling Friday's day of action.
CIE has stated that insurance cover will be extended to all users of buses and trains and to those in stations during the day of action.
It is expected that Irish Rail, Dublin Bus and Bus Éireann will lose a total of at least €1.5 million in revenue on Friday.
Ticket inspectors who are union members will not collect or inspect tickets during the day's action.