Rail union the RMT said it will be holding a protest in London on Friday, April 26, to mark three years since the start of industrial action over driver-only operation.
The RMT campaigned against the changes which make drivers responsible for opening and closing train doors, with guards transferred to the role of on-board supervisor.
Its members took part in more than 40 days of strike action in the dispute, move which caused travel chaos for Sussex commuters.
The ASLEF union which represents drivers joined the RMT in taking industrial action, but eventually accepted a deal from Southern operator Govia Thameslink Railway (GTR).
But the RMT never came to an agreement with the rail operator.
The changes to the guard role were implemented two years ago.
Mick Cash, RMT general secretary, said: "Once again, I would like to congratulate our members for their continuous show of strength and solidarity throughout the long dispute on Southern Railway which is solely about putting rail safety and access before private profit.
“Since the dispute began on Southern, RMT members have been involved in similar campaigns against other companies over attempts to remove the guard from the train. Only on Southern have we been faced with such a stubborn and point-blank refusal to enter serious discussions.
“The union thanks the travelling public for their support and understanding throughout our campaign to put safety and access to services for everyone using our railways right at the top of the agenda.”
A Southern spokesman said: "Since we made the change opposed by the RMT over two years ago, performance and passenger service on Southern have improved, and more trains now run with a second crew member on board. Industry safety bodies have confirmed that having the doors closed by the driver is at least as as safe as by a conductor. We urge the RMT to end their unnecessary dispute."
On April 26, RMT members will gather outside the London headquarters of Keolis UK, a company which owns 35 per cent of Govia Thameslink.
The union said: "The action will attract RMT members from across Britain in a show of solidarity with those involved in the dispute who are continuing to fight to retain a second safety critical person on the train."