The Enterprise train refurbishment has been delayed by six weeks, railway workers said.
Unite the union blamed public transport company Translink for "mismanagement" in overhauling the Belfast to Dublin flagship service.
Unite regional officer Davy McMurray said the three trains were supposed to be back in service at the end of November.
Translink said work was complete and the trains would be running next month.
Mr McMurray said: "Unite has facilitated this work in every way possible. Our members have accommodated management in terms of overtime and flexible working.
"This is a case of management failure and this overrun is resulting in considerable ongoing costs which will have to come out of what is an already hard-pressed budget for public transport services.
"In the absence of the refurbished trains, the Enterprise service is being run with substitute trains mostly without first class carriages and with a limited catering service - all of which is undermining the quality of the service offered.
"Unite is calling on management to act urgently and address this delay."
The first newly refurbished Enterprise train has entered passenger service as part of the £12 million upgrade programme, Translink's website said.
A spokeswoman added: "The refurbishment of all Enterprise trains was scheduled for completion by the end of 2015, with completion actually taking place on December 14.
"The train sets are now in a period of testing and commissioning, to ensure that all equipment operates to specification, in accordance with project timescales. All four sets are expected to be in passenger service by the end of February.
"All costs associated with this refurbishment programme, which is largely funded by Special European Union Programmes Body (SEUPB), are in line with projected budgets.
"The Enterprise remains one of our flagship services and it is our priority to complete the remainder of the work and facilitate the reintroduction of a high-quality refurbished fleet for our passengers."
The redesign includes new livery, replacement of the passenger information system, a new electronic reservation system, security cameras and redesigned interiors.
In 2013/14 almost 840,000 people travelled to Dublin on the Enterprise.