The Rail, Maritime and Transport (RMT) union said drivers it represents on Southern Railway will vote on industrial action over driver-only trains.
The ballot will open on November 4 and close two weeks later, mirroring the voting timetable of the main drivers’ union, Aslef, in the same dispute.
Southern is embroiled in a bitter dispute with the RMT over changes to the role of conductors which has sparked a campaign of industrial action.
General secretary Mick Cash said: “Drivers on Southern Rail will find themselves dangerously exposed if the company decide to plough ahead with their ill-conceived and dangerous plans for a wholesale extension of driver-only operation on their routes.
“Drivers will be placed in an impossible position, trying to monitor despatch from overcrowded platforms while they should be concentrating on the job of driving the train. The consequences for safety at the critical platform/train interface are horrendous.
“The union will be calling for a massive Yes vote as we step up the pressure on Southern to abandon their high-risk plans.”
Southern insists driver-only trains are safe, stressing the system is in place across large sections of the rail network.
Meanwhile, the RMT announced the timetable for a strike ballot among almost 3,500 London Underground workers over jobs and the closure of ticket offices.
Voting will start next week and the result will be known in mid-November.
Mr Cash said: “London Underground knows that it has compromised safety and customer service across its stations by cutting jobs. A responsible employer would reverse the job cuts and put staff back into station control rooms.
“If London Underground really cares about passengers, it would reverse the ticket office closure programme.
“The continuing impact of those cuts leaves us with no alternative but to move to a ballot for industrial action and the terror alert at North Greenwich last week has confirmed the urgency of the current situation and the nonsense of axing station staff.”
A Southern spokesman said: “This ballot is unnecessary. Our proposals to transfer the full operation of the train to the driver to allow the on board supervisor to concentrate on customer service are totally safe, and a proven method of operation.”
London Underground managing director Mark Wild said: “The safety of customers and staff is our top priority and our hard-working people ensure that London Underground remains one of the safest metros in the world.
“There are more staff available to customers in public areas of stations than ever before and crime is at its lowest ever level.
“An independent review of the closure of ticket offices is being conducted by London TravelWatch to ensure that we continue to deliver high-quality customer service.”