The biggest rail workers union is holding a protest as new operators take over one of the country's leading train franchises, as part of a campaign to retain guards on trains.
The Rail, Maritime and Transport union say the First Group/MTR is refusing to give guarantees on staffing as it takes over South West Trains services.
The new operators have insisted there will be a second member of staff on trains.
The protest will be held on Monday outside Waterloo station in London, which has been hit by chaotic scenes in recent weeks because of major engineering works and problems caused by issues such as signal and points failures as well as a minor derailment.
Mick Cash, RMT 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.
"Unfortunately in talks with First/MTR 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.
"Monday morning's demonstration will be the kick-off point for RMT's campaign to keep the guard on South West Trains."
A statement by the new operators 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.
"This is why we will always plan to retain two people on our services.
"During our franchise, we will increase the number of drivers and guards so that we can deliver more services for our customers.
"We are looking to create a secure environment and meaningful career progression for our employees and we have significant plans to invest in learning and development, which we are looking forward to discussing with our employees.
"We will discuss the role of our guards with our staff, and how that role can be enhanced to provide a better service to our customers.
"Our plans are to introduce brand new British-built trains, an investment of £895 million. These trains will cover the Windsor, Reading and London suburban routes, which will mean an enhanced travelling experience for our customers with free reliable Wi-Fi, at-seat charging points, wide gangways and doors, toilets and air conditioning.
"Elsewhere on the network we will refurbish and refresh existing trains.
"We are not committed to any one method of train operation for this new fleet, to be introduced by December 2020, and we will fully involve our employees and trade unions, in the normal way, in discussions on the initial designs of the new trains and ideas on how they best operate.
"We are confident employees will be excited by the focus on higher quality services for passengers that we will bring to this franchise. We look forward to engaging with our employees and the trade unions in the coming months.
"We told the RMT that this is something we wish to discuss further with them in the next few weeks."