Train passengers face weeks of disruption over Christmas and into the New Year.
Here the PA news agency answers eight key questions on what is happening.
-What is the dispute about?
Trade unions representing railway workers at Network Rail and train operating companies have been engaged in a long running row over jobs, pay and conditions.
-Which workers are going on strike in the coming weeks?
Thousands of members of the Rail, Maritime and Transport union (RMT).
-When are the strikes happening?
There are two 48-hour strikes planned for before Christmas, and two in the New Year.
The affected dates in December are Tuesday 13 and Wednesday 14, and Friday 16 and Saturday 17.
The affected dates in January are Tuesday 3 and Wednesday 4, and Friday 6 and Saturday 7.
-Is that it?
No. On Monday night, the RMT announced further strike action at Network Rail from 6pm on Christmas Eve until 6am on December 27.
-What impact will the strikes have on services?
Strike day timetables have not been announced, but passengers are being warned to expect a “very limited service” with “no trains at all on some routes”.
Services are also likely to start late and end early.
-How will the additional strike dates affect maintenance work?
The Christmas period is key for Network Rail to carry out maintenance work so the new strike dates are a major blow for the infrastructure management company.
-What services are normally scheduled over the festive period?
Services normally wind down earlier than normal on Christmas Eve, with no trains on Christmas Day and a very limited timetable on Boxing Day.
December 27 is usually a bumper day for rail travel with many people returning home from visiting loved ones.
-What about the Transport Salaried Staffs Association (TSSA)?
The TSSA has called off its Network Rail strikes planned for December and is putting an offer to its members.