Rail and Tube workers are to be balloted for strikes in separate disputes which could cause travel chaos in the run-up to Christmas.
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. The dispute 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.
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."