Docklands Light Railway (DLR) workers are to be balloted for industrial action in a dispute over jobs and pay.
Docklands Light Railway is run by Serco
More than 250 members of the Rail, Maritime and Transport (RMT) union are being asked if they want to strike.
The union says Docklands Serco, which runs the DLR, wants to cut station staff from 42 to 34 and reduce station assistants' pay by £5,000.
Serco said the changes are part of a reorganisation which will actually increase numbers of staff at stations.
The RMT also claims the company wants to sack or downgrade more than half of the current station supervisors.
RMT general secretary Bob Crow said: "However they dress it up, this re-organisation will result in fewer people on duty on a railway where most stations are already unstaffed, as well as the downgrading of the skills of those station staff that remain.
"It appears that Serco seriously underbid for the new seven-year contract they have just been awarded, but RMT members are determined that they will not pay with their jobs."
But Serco Docklands' managing director Tony Thomas said: "Our organisational change means an increase in station staff and a staff presence at more stations on the DLR.
"Coupled with our new team of Travel Safe Officers, passengers will see more staff than ever on DLR stations and trains, improving customer service and security on the railway."
In a separate dispute, the RMT is threatening to ballot London Underground staff if "lampmen" are not reintroduced within seven days.
Lampmen provide lighting for inspection of particularly vulnerable stretches of track, and were introduced following the Camden Town and Hammersmith derailments.
The union claims they were removed by Tube maintenance company Metronet without consultation or agreement earlier this month.
A Metronet spokeswoman said the lampmen were brought in after the Hammersmith derailment as a short term measure but better methods of detecting defects are now in place.
She added: "Because of the improved pro-active maintenance regime, Metronet has been consulting with the RMT over the last year to remove the lampman from assisting the patrolman, with patrolmen working in pairs in the future.
"The RMT has not informed Metronet of their intent to ballot over this issue."