About 660 jobs are at risk on South West Trains
Workers at three train companies are to be balloted for strike action in a row over plans to cut about 1,000 jobs.
The Rail Maritime and Transport Union move could hit services into London run by National Express East Anglia, First Capital Connect and South West Trains.
More than 3,500 staff at the firms are to vote over the redundancy proposals.
Meanwhile, about 300 RMT members at London Overground are to be balloted over claims that industrial relations have broken down at the company.
South West Trains has announced about 660 job losses, National Express East Anglia plans to cut 300 jobs and 20 positions are going at First Capital Connect following changes to ticket office opening hours.
Voting in all the ballots will take place between 3 and 17 March.
The RMT said job cuts are being planned even though First Capital Connect, South West Trains and National Express East Anglia continue to make healthy profits and it had failed to receive assurances there would be no compulsory redundancies.
South West Trains said it was reviewing its business to focus on customer service and operations amid "difficult economic times".
A spokeswoman said it would do everything it could to find affected staff alternative employment.
First Capital Connect said it was still in consultation with the RMT but there will be no compulsory redundancies and it would like to retain staff whose jobs were at risk.
On the London Overground dispute over restructuring proposals, Transport for London said: "We would urge that the dispute is settled without the need for industrial action and disruption to passengers."