The biggest rail union has confirmed it will meet Southern Railway for talks despite attacking a deal between the company and Aslef.
The Rail, Maritime and Transport union described the agreement between Aslef and the company over driver-only trains as a "shocking betrayal" of staff and passengers.
RMT leader Mick Cash has written to Charles Horton, chief executive of Southern's owners Govia Thameslink Railway, saying he will take part in fresh talks over a long-running dispute about the role of conductors.
Mr Cash also sent a circular to RMT members criticising the Aslef deal.
Mr Cash told his members: "The GTR deal is bad news for train crew and passengers.
"You have lost the guarantee of a second member of staff on your train. Drivers will be on their own and exposed and vulnerable.
"We already know that GTR have been running many trains without on-board supervisors (OBS) and the exemptions in Appendix A of the deal will make this even more frequent and represent the thin end of the wedge leading to the de-staffing of our trains.
"OBS will not be safety critical, despite what was said at the conclusion of the talks.
"They will only be trained to 'undertake the relevant specific degraded safety critical task'.
"So the OBS, even if one is on a train, will not be trained and passed out in the safety critical responsibilities that a guard/conductor has, including track safety.
"With no guarantee of a second member of staff the deal is a disaster for disabled passengers and will see even more disabled passengers left behind on platforms.
"The deal allows for the extension of DOO (driver-only operation) to nearly all Southern lines."