The new South Western rail franchise holder has been criticised by a union for allegedly refusing to guarantee a second staff member on board its forthcoming trains.
FirstGroup takes over on Sunday amid ongoing disputes over driver-operated doors and the role of train guards.
The company said it would be "retaining a second person" on board trains.
The RMT union said in talks the firm refused to provide a guarantee "despite what they have been telling the press".
FirstGroup and Hong Kong company MTR will provide 90 new trains by December 2020 with doors that can be operated by the driver.
Franchise managing director Andy Mellors said: "Our commitment with the 750 new carriages for the suburban routes out of Waterloo is that we will be retaining a second person on board those trains."
But following talks the RMT said FirstGroup altered the wording to say it was "planning" to keep a second person.
General secretary Mick Cash said: "The company refused to give a guarantee that there will be a second person on their trains despite what they have been telling the press.
"They also failed to clarify what safety competencies and role a second member of staff would have on their services."
The union sees a second member of staff on board its trains as "critical" to passenger safety.
In an earlier BBC interview, Mr Mellors said the new trains would "retain a second person" on board, but could not say what that person's role would be.
A company statement said: "We know that a second member of staff provides assurance to our passengers on trains and ensures we have capable colleagues on board to deliver assistance during the journey for anyone that needs help or advice while travelling on our trains."
Disputes over driver-only operated trains on other rail services have led to 16 months of industrial action by the RMT.
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.