Rail managers are to stage a three-week ban on weekend overtime which they warn threatens to disrupt travel across the network.
More than 2,000 members of the Transport Salaried Staffs Association will launch the action on Friday in protest at a below-inflation pay offer.
"It is expected that this action will have a significant impact on weekend engineering work being carried out over the next three weeks," said union leader Manuel Cortes in a message to members at NR.
He warned that the action would be stepped up unless NR management agree to talks by September 22.
An NR spokesman said: "While we understand the TSSA's position is to push for more money for their members, we have a responsibility to drive through savings, especially as it would be British taxpayers that would foot the bill for any big wage settlement.
"This action will have no impact on train services as the numbers involved represent just a small proportion of our management grades."