Rail firm Hitachi says it has contingency plans in place to deal with a strike by some of its workers.
Members of the Rail, Maritime and Transport union walked out on Sunday for three days in a dispute over pay and conditions.
Hitachi said only 5% of its maintenance workforce were taking action.
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A Hitachi Rail spokesperson said: “We are disappointed at RMT’s decision to carry out industrial action, especially as the industry is attempting to encourage people back onto the railways.
“We will continue to work with RMT to find a solution and urge them to continue the negotiations.
“In the meantime, we have contingency plans in place to mitigate any potential impact on passenger service and ensure the highest levels of safety are maintained.”