Workers will walk out from midnight on Monday 30 March
Workers at two rail companies operating across the South East and East Anglia are to strike for 24 hours on Monday.
About 1,500 staff at First Capital Connect and National Express East Anglia will walk out from midnight.
The Rail Maritime and Transport union (RMT) ordered the strikes, accusing the two firms of failing to guarantee no compulsory redundancies would be made.
The strikes coincide with the second day of a 48-hour stoppage by 100 conductors at London Midland.
The conductors' strike is about a long-running dispute over Sunday working.
RMT secretary Bob Crow said First Capital Connect and National Express East Anglia, which run commuter services into London, were seeking to cut jobs "solely to maintain profits and dividends".
He said: "Now our members - and the public they serve - are faced with the prospect of 300 job losses, cuts to services and the creeping casualisation of professional railway jobs with the use of agency and contract labour.
"Yet the National Express empire last year saw its UK rail profits jump by 28% - contributing to a £60m payout to shareholders.
"First Capital Connect is determined to slash booking office opening times at more than 40 stations by 800 hours a week, despite revenue growth of 8% and a £55m dividend."
A National Express East Anglia spokesman said: "We are disappointed that the RMT has called a 24-hour strike despite the fact that only a minority of the RMT members within our employment voted for strike action.
"The RMT are pursuing a needless dispute which will only result in loss of earnings for their members.
"In the event of strike action taking place, we will run the majority of our train services on the National Express East Anglia network."
First Capital Connect said it would be holding talks with union officials.
A spokesman said: "Our proposals do not see any compulsory redundancies to the 13 roles affected and see ticket offices remaining open at peak hours of ticket sales.
"We have worked on plans to ensure that any disruption to customers will be kept to a minimum.
"The RMT is one of four unions recognised by First Capital Connect, which means that the majority of our staff will be available for work during the strike period."