The new South West Trains rail franchise holder is facing its first row with the Rail, Maritime and Transport (RMT) union on its first full day of service as the union protests about the role of the guard.
The union arranged a demonstration for the morning of Monday 21 August at Waterloo station, which is currently dealing with disruption due to major Network Rail upgrade work meaning the closure of platforms one to 10.
The new railway franchise is now called South Western Railway and replaces South West Trains, operating across London and the south of England.
Mick Cash, the RMT's general secretary, said:
There is an agreement in place that there will be no extension of driver only operation on South West Trains and that agreement transfers with the undertaking to First MTR.
All that RMT is seeking is a clear assurance that there will be no interference with the current agreement and that it will be honoured by the new franchise holder.
The company has said it will be "retaining a second person" on board trains, though the union said in talks, FirstGroup hadn't provided a firm guarantee.
FirstGroup and Hong Kong company MTR took over the franchise on 20 August and will provide 90 new trains, that will be rolled out from 2019, with doors that can be operated by the driver.
They were given the go-ahead for the franchise last week, when the Competition and Markets Authority accepted price cap proposals from the companies to tackle competition concerns.
It had raised concerns over one route in particular, as FirstGroup already operates the Great Western Railway (GWR) franchise, running the sole other train service between London and Exeter. So, the firms offered a price cap "on unregulated fares between London and Exeter on both South Western and GWR services".
Tim O'Toole, FirstGroup's chief executive said the new franchise will result in "a step change in customer experience with new and better trains, more seats and services, quicker journey times, improved stations and flexible fare options".
The RMT has been in dispute with a range of train operators over the role of the guard and so-called driver only operation, with a ballot opening today on Greater Anglia after the union said the firm had failed to give a guarantee on the role of the guard throughout the length of the franchise.
And last week, the RMT announced fresh strike dates on Southern rail, Merseyrail and Arriva Rail North. It has been in a long-running dispute with Southern over the role of the guard, with passengers suffering disruption for over a year.