Train crews on the Tyne and Wear Metro are to strike for two days in January.
The action by the RMT union follows a earlier two-day strike and an ongoing overtime ban after it, along with Aslef, rejected a 15% pay increase and changes to rostering arrangements.
Operator Nexus said if it went ahead no trains would run on 6 and 7 January, and urged the RMT to enter into talks.
The RMT said it was keen to end the dispute, but only if any deal was right.
'Hit them hard'
Nexus said the days chosen for strike action include the first days of new school and college terms, and accused the union of "showing a disregard for the travelling public".
Services director, Chris Carson, said: "Nexus has contacted ACAS and we are ready to hold talks through that body, if the RMT call off this unnecessary strike action."
RMT regional organiser, Mickey Thompson, said: "I'm trying to negotiate with the employer, who have not reached out to solve the impasse.
"The only option available to us is to withdraw our labour, our only option to get them back to the table is to hit them as hard as we can."