The strike by members of the drivers' union Aslef at 16 train operators in England was to coincide with the final day of the Conservative Party's annual conference in Manchester.
Many parts of the country will have no services, with those that do run starting later and finishing earlier than usual.
Operators warning of no services include Avanti West Coast, CrossCountry, Northern, Southeastern and TransPennine Express.
Drivers are also banning overtime this week, which will cause further disruption.
The train drivers’ strike, their second in a week, coincides with the longest joint walkout by junior doctors and consultants in England, resulting in “Christmas Day” levels of staff until 7am on Thursday.
Another planned strike by London Underground staff planned for Wednesday was called off after “significant progress” in negotiations, the RMT Union said.
Mick Whelan, Aslef's general secretary, said the government was preventing a settlement to the year-long dispute with train drivers.
He said: "Our members have not had a pay rise for four years – since 2019 – and that's not right when prices have soared in that time.
"Train drivers, perfectly reasonably, want to be able to buy now what they could buy four years ago."
A Rail Delivery Group representative said: "There is a deal on the table for Aslef that would take average driver salaries to £65,000 [$78,440] for a four-day week – that's more than double the average UK salary – and many drivers top up their income further by working overtime.
"We are ready and willing to talk to Aslef's leaders so we can end this damaging dispute but any talks about pay also need to address working practices that date back decades.
"The industry depends on a monthly injection of up to £175 million from the taxpayer – because revenues are still 30 per cent below pre-pandemic levels – while simultaneously facing unprecedented changes in customer travel patterns.
"This isn't just costing taxpayers, it's costing businesses eye-watering sums and all because Aslef's leadership refuse to discuss much-needed changes to ways of working.”
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He said managers, rather than unions, should be in charge of planning shifts.
“It means allowing managers to respond to unexpected staff absences so they can reduce the last-minute cancellations that so frustrate our customers,” he said.
"It means giving our customers more reliable train services when they actually want to use them – particularly on Sundays. That is how any industry survives and thrives."
Doctors have vowed to prolong their strike action in the stand-off with the government over pay.
British Medical Association (BMA) council chairman Prof Philip Banfield said: "We will strike until the next general election and beyond if that is what it takes.
"But our patients need the PM to meet with us now; restore the value of pay now; make a credible offer now; end these disputes now."
Other sectors, including teachers, have ended their disputes with the government after accepting pay offers.