The parent company of Southern Railway has issued proceedings in the High Court, seeking an injunction to stop planned industrial action by drivers.
Aslef is planning strikes and an overtime ban in the run-up to Christmas and a week-long walkout in the New Year (January 9-14).
The strikes would cripple services on Southern, which is already being hit by a guards' dispute, staff shortages and other problems.
The train operator has lodged a court application citing its belief that Aslef's industrial action breaches customers' rights under EU law.
Charles Horton, chief executive of Govia Thameslink Railway, said: "We launch legal action reluctantly but now without any other choice.
"Obviously we would prefer to resolve this directly with Aslef. We asked the union to withdraw the industrial action and to re-enter discussions but they refused to do so, which means that we now have no choice but to go to court."
Aslef is opposed to the extension of driver-only trains.
Mr Horton continued: "We have a responsibility to our customers to do all we can to protect their interests and maintain services for them.
"Passengers now face the prospect of 40 days of continuous industrial action by Aslef and, on top of months of travel misery they've already suffered, it is totally unacceptable.
"The proposed industrial action called by Aslef is unjustified, unnecessary and we believe unlawful. It will severely disrupt all our customers, including the tens of thousands of customers a day who use our services for travelling to and from Gatwick Airport.
"We believe that the industrial action breaches our and our customers' rights under EU law and so we are seeking an injunction to stop the industrial action.
"It's perfectly safe for the driver to have sole responsibility for the operation of a modern train, and that's how a third of the trains up and down the country - with the full agreement and support of Aslef - already operate today."
GTR said, following the application to the High Court, the matter is likely to be considered at a hearing in the coming days.
Until the legal action is concluded it will make no further comment.