The boss of the new South Western rail franchise has refused to commit to protecting the role of train guards, despite calls from the RMT trade union.
FirstGroup and Hong Kong's MTR takeover on 20 August and will provide 90 new trains with doors that can be operated by the driver.
Driver-only operated trains on Southern Rail services has led to 16 months of industrial action by the RMT.
The union has called a meeting with FirstGroup on 17 August.
The South Western rail service currently operates routes between London Waterloo, Reading, Bristol, Exeter, Weymouth, and Portsmouth, as well as Island Line on the Isle of Wight.
At the meeting RMT said it would "remind" FirstGroup that there was a "long-standing agreement in place" with previous owners Stagecoach not to extend the role of the driver or threaten the roles of the guards.
In a BBC interview, franchise boss Andy Mellors confirmed its fleet of new trains would "retain a second person" on board, but refused to say what that person's role would be.
"It's far too early to say exactly how we will be operating those trains. As new trains they can be operated in a variety of ways," he said.
He added FirstGroup and MTR were "committed to recruiting additional staff" as it has pledged to run more trains in and out of Waterloo by December 2020.
The consortium will invest £1.2bn over seven years.