The train drivers’ union ASLEF is due to announce the result of a vote at lunchtime on whether to accept a pay offer from Southern.
The ballot result will be declared hours after on-board supervisors – formerly guards – belonging to the RMT union start a 48-hour strike.
Southern said that most services would run as usual although it had already cancelled some services.
The train company has offered drivers a 28.5 per cent rise over five years, taking salaries up about £14,000 a year to about £63,000.
The union recommended the deal – the third time that it has done so this year.
On the previous two occasions, drivers turned down the offer in the long-running dispute over the driver-only operation of trains.
A number of drivers – like the RMT – have said that the dispute is about safety not money and dislike the way change has been imposed rather than negotiated.
Without guards, there are concerns that disabled people will find it harder to use the railways.
And some drivers dislike the way that Southern – and its parent company Govia Thameslink Railway (GTR) – has tried to divide the unions by holding talks with one union at a time.
But the ASLEF strikes brought the network to a halt while bosses have largely managed to keep services running during the RMT strikes which started more than 18 months ago.
There is a widespread belief that civil servants at the Department for Transport (DfT) and Conservative ministers are behind cost-saving changes which threaten jobs.
Southern has promised not to make any former guards redundant during the remaining years of its operating agreement with the government which ends in 2021.
The company has also set out the exceptional circumstances when trains would run without a second person on board. These might include a guard not having arrived on a connecting service.
Drivers – as elsewhere – would take responsibility for closing train doors safely before leaving a station.
Previously guards – or conductors – closed the doors but a new generation of trains, with CCTV in the drivers’ cab, have been configured so that drivers operate the doors.
As the dispute has continued, Southern has promised to keep a second person on each train unless there are exceptional reasons.
ASLEF general secretary Mick Whelan said: “The proposed agreement on DOO (driver-only operation) means we will have a second safety-trained person on every train covered by this agreement except in exceptional circumstances.
“That person will have all the relevant safety competence including the skills to evacuate passengers in an emergency.”
As the RMT started a 48-hour strike, Southern said: “Services on most Southern routes will operate normally but there will be some alterations.
“The full Thameslink service is expected to operate on all routes.
“Gatwick Express services will not be affected by the strike but please note that some trains have been removed for the leaf-fall period.”
RMT general secretary Mick Cash said: “RMT members stand solid, united and determined again this morning in the latest phase of strike action in a raft of separate disputes which are about putting safety, security and access to transport services before the profiteering of these rip-off private rail companies.
“Political and public support is flooding in as our communities choose to stand by their guards against the financially and politically motivated drive to throw safety-critical staff off our trains.
“The union salutes the members who are standing firm this morning for a safe and accessible railway for all.
“It is frankly sickening that Chris Grayling (the Transport Secretary) and his supporters are prepared to sit back and cheer on overseas operators who are robbing British passengers blind while sacrificing basic safety standards in order to subsidise transport services in Paris, Amsterdam and Hong Kong.
“It’s time for the government to lift the dead hand which is preventing rail companies from negotiating deals like the ones we have successfully struck in Wales and Scotland that guarantee a guard on the trains.
“If it’s good enough for Scotland and Wales it’s good enough for the rest of Britain.
“Again this morning I am calling on Theresa May and Chris Grayling to call off the centrally imposed blockade on serious talks in these disputes and allow us to get on with genuine negotiations with their contractors.”
