Workers on two rail networks have begun a 24-hour strike over the future of guards on trains.
Members of the RMT transport union stopped work at midnight on Northern Rail and South Western Railway (SWR).
Northern said it would run 30% of its services, with "very few trains" before 09:00 BST and after 18:00.
South Western said it would operate half of its normal service. Both companies said some routes would have no trains or replacement buses.
The RMT (the National Union of Rail, Maritime and Transport Workers) is asking for a guarantee of guards on trains and a halt to the "rolling out of driver-only operation".
SWR said it would roster guards on all services but trains could still operate in "exceptional circumstances" if no guard was available.
Northern said it was prepared to "guarantee jobs for conductors".
The Northern strike is the third in a series of walkouts planned for six consecutive Saturdays until 29 September.
A further strike on SWR is planned on 15 September.
RMT said it had secured "guard guarantees" in Wales and Scotland as well as on a number of English franchises.
What is the dispute about?
- The strike is part of a nationwide two-year dispute about the increase of driver-only-operated trains
- Driver-only-operated trains are where the driver, rather than the conductor, opens and closes the doors
- A third of Britain's services already have this in place and it has been in operation for about 30 years
- The rail safety regulator says it is safe - a position that has been supported by the government
- Rail unions disagree - they say the on-board conductor or guard has a much better view of the doors and can stop people getting trapped
- The London tube network is driver-only operated