Workers on Northern Rail and South Western Railway (SWR) are staging the latest in a series of 24-hour strikes.
Members of the RMT transport union stopped work at midnight as part of a two-year dispute with rail firms over the future of train guards.
Northern said it would run 30% of its normal services while SWR said more than half of trains would operate.
The RMT (Rail, Maritime and Transport) union is calling for a guarantee that a guard will work on all services.
SWR said it would roster guards on all services but trains could still operate in "exceptional circumstances" if no guard was available.
Northern said the issue would be discussed with the union on Wednesday at the ACAS conciliation service.
The Northern strike is the fourth in a series of walkouts planned for six consecutive Saturdays until 29 September.
The union said it would meet next week to consider further strike dates on SWR.