Only about one in five trains will run on Wednesday, across about half of the network, with some areas seeing no trains at all throughout the day.
Some 40,000 British railway workers went on strike on Wednesday over pay and jobs, a month after their biggest strike in 30 years, in the midst of the UK's purchasing power crisis.
The RMT rail union called a 24-hour walkout after a historic three-day strike at the end of June, hoping for better pay in the face of soaring inflation in the country, which could exceed 11% by the end of the year.
This is a burning issue that the successor to Prime Minister Boris Johnson, who resigned on 7 July after a series of scandals and lies, will have to tackle. Chief diplomat Liz Truss and former finance minister Rishi Sunak are the two finalists in the race to succeed him.
Due to the strike, only about one in five trains will run on Wednesday, across about half of the network, with some areas seeing no trains at all throughout the day.
The strike has also affected Eurostar trains, causing cancellations and timetable changes.
In addition to this action, the RMT and TSA unions will launch coordinated strikes on 18 and 20 August and the RMT has announced a strike on the London Underground on 19 August.
RMT general secretary Mick Lynch said his union's members were more determined than ever to get higher pay, job security and good working conditions.
He said the state-owned Network Rail had made "no improvement on its previous pay offer".
Transport minister Grant Shapps attacked the unions, accusing them of increasing strikes and threats of strikes to the detriment of thousands of users.
"We need to do more to stop these very militant far-left unions from disrupting the daily lives of ordinary people," the minister said on SkyNews on Wednesday.
The strike is likely to make it harder for fans to get to the Women's Euro semi-final when Germany and France meet at Stadium MK in northwest London on Wednesday night.