Passengers should not travel by train unless absolutely necessary on Saturday due to the anticipated disruption from strikes, the rail industry has said.
Only a fifth of trains are scheduled to run on Saturday, with services starting late and finishing early.
This is the eighth day of action taken by the union over pay and conditions.
National Rail Enquiries warned customers that it is inevitable that services will be cancelled or severely disrupted. There will be very limited service on Saturday 8 October with no service at all in some places. If you travel by train if you are absolutely necessary, plan ahead and check before you travel. Passengers with advance, off-peak or anytime tickets affected by the strikes can use their ticket on Friday, or up to and including three days later. Rail operators said so.
People can change their tickets to travel on an alternate date or get a refund if their train is cancelled or rescheduled.
Network Rail's signalling staff strike across England, Scotland and Wales will cause RMT strikes to be widespread, so only a fraction of usual services will run, even though train operators are not directly involved in the dispute.