Strikes by Southern Railway guards and drivers in the RMT will go ahead next week after its "exclusion" from talks between Aslef and the company, the union announced.
Guards in the Rail, Maritime and Transport union will walk out for 24 hours on Monday in their long-running dispute over changes to their role.
The union also has a small number of members who drive Southern trains, and they will strike on Tuesday, Wednesday and Friday next week.
The main drivers' union, Aslef, has suspended its planned strikes next week, and called off an overtime ban, while it holds talks with the train operator under the co-chairmanship of the TUC.
RMT general secretary Mick Cash said: "RMT demands again today that this union be given access to exactly the same talks process in our Southern Rail disputes as has been brokered for our sister union by the TUC.
"We would remind all parties that not only is RMT a recognised drivers' union on Southern but that it is also our guards members who have made huge personal sacrifices in the long fight for passenger safety.
"The notion that some sort of deal can be done which leaves those Southern guards out in the cold is ludicrous.
"As a result of our exclusion from the talks process set up by the TUC and the Government both our guards and drivers' action remains on next week and we demand and expect a positive response to our fresh demand for a seat at the negotiating table."
A Southern spokesman said: "The talks that are now taking place under the auspices of the TUC are as a result of a suggestion by Aslef, which we welcomed and accepted in order to find a way to end their drivers' dispute.
"As we have stated, we are happy to have formal talks with the RMT too when they're ready to do so, and lines of communication with them are open.
"In the meantime, to show good faith, we would ask them to follow Aslef's lead and suspend their one-day conductors' strike planned for next Monday."
Talks between London Underground, the RMT and TSSA over the Tube dispute have adjourned and will resume on Monday, said Acas.
Talks between Aslef and Southern Rail were adjourned until tomorrow.
Co-chairs Frances O'Grady and Andy Meadows said: "We have made further progress in talks today and we will be reconvening tomorrow."
Southern said it will run more than 70% of its trains next Monday - 10% more of its normal timetable than it operated on previous conductor strike days.
"This follows the completion by Southern at the start of the year of its programme to put drivers in sole control of the operation of the train, including closing the doors, on nearly 80% of routes and the transfer of many conductors to the new on-board supervisor role focused exclusively on customer service," said a statement.
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