Tens of thousands of commuters in Dublin have been forced to seek alternative transport as the Luas ground to a halt in a staff row over pay.
Workers said they had no option but to strike after calls for last-ditch talks to avert the industrial action failed.
The stoppage is expected to last until Saturday, hitting about 90,000 passengers each day on Thursday and Friday.
Trade union Siptu has attacked Transdev, operator of the light rail system, accusing management of refusing to offer anything other than inflation rate pay rises.
Workers’ representatives, who have been championing salary hikes of 8% to 53%, claim pay scales mean drivers can work for nine years and hit a salary of 42,247 euro or 47,941 euro for traffic supervisors.
Transdev said the pay claims would cost the company more than 20 million euro over five years.
Transport Minister Paschal Donohoe said he was powerless to offer more money to meet the demands.
Normal service is not expected to resume until 6.30am on Saturday morning.
Transdev has apologised to customers for the inconvenience.
A repeat two-day strike is planned for next week, on February 18-19.