The head of train drivers’ union Aslef warned it “won’t blink” about taking further strike action as it prepares for today’s national rail stoppage.
Mick Whelan, general secretary of Aslef, told the Mirror: “We are in this for the long haul.”
No trains will run between London and Britain’s biggest cities as multiple unions combine strikes.
Overall, just 11% of rail services are expected to operate.
Rail passengers are being urged only to travel if necessary.
The near shutdown of the network is set to hit people travelling to the capital for tomorrow’s London Marathon, as well as next week’s Tory Party conference in Birmingham.
Members of the Rail, Maritime and Transport union (RMT), Aslef, Unite and the Transport Salaried Staffs’ Association (TSSA) will stage a co-ordinated walkout.
It will be the first time the unions have walked out on the same day, so services will be more significantly disrupted than on previous strike days.
Trains will start later in the morning and finish earlier in the evening, and there will be no trains at all across large parts of the network.
Unlike previous strike days, today there will be no trains between London and a number of other major UK cities - including Edinburgh, Manchester, Liverpool, Birmingham, Newcastle, Brighton and Norwich.
Around 9,000 members of Aslef at 12 rail companies will strike today, in the union’s largest day of action yet.
A walk-out by members at 14 rail firms is scheduled for next Wednesday.
Mr Whelan said: “We are still meeting in good faith”.
However, he added: “The solution is still a long way off.”
Acknowledging the impact on travellers, he said: “Every one of our members have family that want to travel, want to go to the football, or whatever.
“We don’t want to go on strike – withdrawing our labour, although a fundamental human right, is always a last resort for this trade union – but the train companies have been determined to force our hand.”
Asked if it was preparing for more strike days, Mr Whelan said: “We won’t blink to call more.
“We are in this for the long haul.”
RMT members are also due to strike next Saturday.
Passengers are also advised that there is likely to be some disruption in the early morning of tomorrow as workers return to duties.
Andrew Haines, Network Rail chief executive, said: “Despite our best efforts to compromise and find a breakthrough in talks, rail unions remain intent on continuing and coordinating their strike action.
“This serves only to ensure our staff forgo even more of their pay unnecessarily, as well as causing even more disruption for our passengers and further damaging the railway’s recovery from the pandemic.”
Daniel Mann, director of industry operations at Rail Delivery Group, said: “These strikes are unnecessary and damaging.”
Transport for London warned its services will be affected by the strikes, with no service expected on London Overground today and next Wednesday.
Some of London Underground and the Elizabeth line will also be affected on both days.
Essential engineering works mean no Piccadilly line service to Heathrow on Saturday 1 and Sunday 2.
Most of the public transport network will operate as usual, but customers were advised to check before they travel and leave more time for journeys.
Runners and spectators trying to get into London in time for the 9.30am start of the London Marathon in Greenwich on Sunday were warned they were likely to be frustrated by the strike.
Trains travelling any reasonable distance into central London will not arrive much before 9am.
