Hundreds of guards at South Western Railway have begun voting in a strike ballot in protest at driver only operation.
The RMT says the new company has plans to change the role of its members.
The union say safety will be at risk.
SWR says it has no plans to cut staff but hasn't ruled out changes to working practices.
The ballot result is due later this month.
The result of a strike ballot among Southern Railway drivers will be announced today, raising the threat of travel chaos in the run up to Christmas.
Members of the drivers' union Aslef have been voting on whether to launch a campaign of industrial action in a dispute over driver-only trains. The union's executive will meet to discuss the outcome of the ballot before deciding the next move.
The row is separate from the long-running dispute between Southern and the Rail, Maritime and Transport union over changes to the role of conductors. Aslef accused the company of wanting to impose changes rather than reach an agreement.
Aslef will have to give seven days notice of any industrial action.
Members of the RMT have staged a series of strikes on Southern in recent months, with three-day walkouts due to start on December 6, 22 and 31.
Southern services are also being disrupted on other days because of issues including a shortage of staff.