Network Rail has been granted an injunction preventing a crippling national strike by signal workers.
Judge Mrs Justice Sharp made an interim order against the Rail Maritime and Transport union at a hearing in London's High Court.
Earlier, a QC representing Network Rail told her that "unlawful" strike action would cause "immense damage to the economy".
The union, which has called four days of action from next Tuesday, the day after the Easter break, strongly contested the legal move.
Bob Crow, general secretary of the RMT, said: "This judgment is an attack on the whole trade union movement and twists the anti-union laws even further in favour of the bosses.
"Workers fighting for the principle of a safe railway have had the whole weight of the law thrown against them. Our executive will meet this evening with a recommendation for a re-ballot."
