Southern rail bosses are to take the Aslef union to the Supreme Court over its industrial action which has halted hundreds of daily services.
The move came on the second day of strike action this week by the union in a bitter dispute over driver only trains.
A further strike will be held on Friday as well as three more walkouts at the end of the month.
The strikes have halted most of Southern's 2,200 daily services.
The company's owner, Govia Thameslink Railway (GTR) lost a court case and an appeal last year to try to stop the strikes.
A statement said: "GTR is determined to protect its passengers and its business from unlawful industrial action.
"GTR is therefore prepared to continue its legal claim to the Supreme Court, as it believes that it has an arguable case that the industrial action is unlawful under EU law."