Workers on Northern and South Western Railway (SWR) have begun 24-hour strikes on the last Saturday before Christmas.
The walkout, from midnight, is part of the long-running dispute over the future of train guards.
Northern said the strike, by the Rail, Maritime and Transport Union (RMT), would disrupt markets and other events "at a vital time for businesses".
The RMT said its action was in defence of a "safe and accessible" railway.
General secretary Mick Cash said he would not allow "the dilution of the safety culture on Northern trains in a drive to prop up Deutsche Bahn's profits as [Northern's] parent company hits the skids".
Northern said the walkout would harm Christmas traders and "damage the economic well-being of the north of England".
It said very few of its services would run after 17:00 GMT.
SWR said the union was targeting people "trying to travel to be with family and friends... at what should be a time of relaxation and enjoyment".
It said a "reduced service" would operate, although some routes would have no trains or replacement buses.
Previously the union called for a guarantee that trains will not run if no guard is available, while SWR has said they would run in "exceptional circumstances".
The RMT has been involved in a two-year dispute with several rail companies over the issue.
It previously said it had secured "guard guarantees" in Wales and Scotland as well as on a number of English rail franchises.
Further strikes have already been announced on SWR on 27 and 31 December.
Walkouts on Northern are planned on each Saturday until the end of January.