If necessary, people are being told to only travel by train.
The Rail, Maritime and Transport union will walk out on Wednesday.
Talks over pay, jobs and terms and conditions failed after the biggest rail strikes in 30 years.
Some places will not have trains at all on Wednesday.
There won't be any trains in or out of the following areas:
Network Rail has published a special timetable for Wednesday which will allow trains to start later and finish earlier.
The last trains from London will leave for Edinburgh at 14.
Mick Lynch said the union would keep trying to reach a deal with Network Rail.
"If we could get a breakthrough then we wouldn't have to take strike action, but there is a big gap between the parties at the moment," he said.
We are constantly in dialogue with all of the elements of the industry.
The strike will affect all train operators, including Transport for London.
The disruption is expected to have a knock-on effect on Thursday. The strikes are scheduled for 18 and 20 August.
The strike on Wednesday will affect travel to the Women's Euro 2022.
There will be a separate strike by the train drivers' union on Saturday.
The English Football League season starts today.
Despite our best efforts to find a breakthrough, I'm afraid there will be more disruption for passengers this week as the RMT seemshell-bent on continuing their political campaigning, rather than compromising and agreeing a deal for their members."
Passengers with an advance, off-peak or anytime ticket can travel the day before or up to 2 August.
If the train is canceled or changed, they can change their tickets to another date.
The delay repay scheme can be used by season ticket holders who don't travel on 27 or 30 July.