The entire rail network will come to a standstill next month, with passengers facing three strikes in the space of a week.
Action by the Aslef union was already confirmed for Oct 1, when the RMT will also strike, and Oct 5.
The combined action, taking in Network Rail and train operators, is expected to cause severe disruption for thousands travelling to and from Birmingham for the Conservative Party conference.
Network Rail warned of “very significant disruption” for passengers. It confirmed it would once again ask people “to only travel if absolutely necessary” amid reduced service levels.
'Get back to work,' says Liz Truss
The Prime Minister had pledged to bring in the legislation within 30 days of taking office.
However, following the period of national mourning for the Queen, she admitted that the deadline may have to be pushed back to as late as the end of November.
Speaking in the US on Wednesday, the Prime Minister said: "We are committed to bringing in legislation for minimum service levels on rail as soon as possible.
“My message is: I want this country to be successful. And that means people being able to get to work. People being able to get on with their business, people being able to move projects forward.
“So I would encourage rail workers to get back to work. There’s no doubt we’re facing tough times as a country. I want to take a constructive approach with the unions, but I would tell them to get back to work.”
The Department for Transport warned that the unions were taking the railway system back to the "dark ages".
A spokesman said: “Thousands of people once again will have their day-to-day lives disrupted and be unable to attend work, school or doctor’s appointments.
“Our railway is in desperate need of modernisation but all more strikes will do is take it back to the dark ages and push passengers further away.
“We urge union bosses to reconsider this divisive action and instead work with their employers, not against them, to agree a new way forward.”
'No choice' but to strike
Union mandarins were urged to rethink by the Government after an introductory meeting between Anne-Marie Trevelyan, the Transport Secretary, and Mick Lynch, the secretary-general of the RMT.
No progress was made in a separate meeting between Ms Trevelyan and Mick Whelan, the Aslef general secretary.
Grant Shapps, Ms Trevelyan’s predecessor, had refused to meet union bosses during his time at the Department for Transport.
Mr Lynch welcomed the “more positive approach” taken by Liz Truss’s administration and said he would negotiate “in good faith”. However, he claimed members had “no choice” other than to strike, as no new pay offer had been made.
George Freeman, a former transport minister, said: “The rail strike disruption to millions of hard-working people in public services and business will rightly be greeted with dismay and a lot of anger at union fat cats playing politics with people’s livelihoods in a national economic crisis.”
Greg Smith, the Tory MP for Buckingham, who sits on the House of Commons transport committee, said the unions “keep trying the same trick” and are “hoping the end result will be different”.
He added: “It’s quite clear that they’re not going to win and all they’re doing is damaging a fragile UK economy, with the latest strikes hitting people going out.
“All they’re going to do is drive more and more people away from the railways and threaten their very jobs and existence in the first place.”
Tim Loughton, another Tory MP, added: “Given that the new Transport Secretary has already taken the initiative to promote a fresh approach and new talks, this rush to yet further industrial action smacks more of political manoeuvring than a genuine concern for the welfare of rail workers.”