Rail staff from three firms across England have started 24-hour strikes in a dispute over the role of guards.
The RMT's 30th strike day in its dispute with Southern over plans to make trains driver-only-operated (DOO) has spread to the north of England.
Guards and drivers working for Merseyrail and Northern are taking action over similar DOO proposals.
Management at all three companies say they want a negotiated solution and deny jobs will be lost.
Live updates: The latest from Northern, Southern and Merseyrail
Bosses at Merseyrail made a late attempt to derail the action on Thursday, seeking an injunction against the dispute, which was rejected by the High Court in London.
Disruption on Monday
- Southern: A number of its train services will not run - including between Clapham and Milton Keynes and London and Brighton.
- Merseyrail: A reduced service will run with trains every half hour between 07:00 and 19:00 instead of every 15 minutes. The company said timetables were likely to change during the strike.
- Northern: It is running only 40% of its normal services with trains running on its busier routes between 07:00 and 19:00. The firm said it would run 300 rail replacement bus services.
Merseyrail members have been refusing to work "rest days" since Monday.
RMT general secretary Mick Cash said the union's position on DOO was "perfectly clear" and added it would "not agree to any introduction of DOO".Image copyright PA Image caption The dispute is over who is responsible for opening and closing the train doors
Arriva Rail North, which operates Northern trains, provides services across the north of England.
Northern proposes to modernise its network by 2020 with the introduction of 281 new carriages, 243 upgraded trains, 2,000 extra services each week and better stations across the network.
It has promised to protect jobs and pay and said it is "disappointed" by the strike.
Analysis: Richard Westcott, BBC Transport Correspondent
It's the South and the North today, but could this dispute keep spreading across England?
The government's made it clear that it wants an expansion of "driver-only-operated" trains and that puts it at loggerheads with the unions.
The Department for Transport talks about introducing them in the next two franchises about to be awarded, South Western and West Midlands.
The unions say it's a long-term ploy to get rid of all train guards and save money - they claim it puts passenger safety at risk.
But rail bosses argue it's about modernising the service, freeing up the second on-board person to deal with passengers rather than closing the train doors.
Handing all the safety jobs to the driver means you don't HAVE to have two people on every train before it can leave the station. That would shrink the power of the RMT, because more trains would be able to run if their guards went on strike in the future.
Merseyrail plans to introduce a new fleet of 52 (DOO) trains from 2020 and said none of the permanent guards or guard managers would lose their jobs.
The company said it has "pledged to do everything we can to bring the dispute to a satisfactory and swift conclusion".
Southern has said the union was "hell-bent on further strike misery".
Its parent company Govia Thameslink Railway (GTR) said the union chosen to put its members "through even more pointless industrial action."
All three strikes are due to end at midnight.Image copyright PA Image caption Southern rail passengers have faced months of delays and disruption Image copyright Merseytravel Image caption Merseyrail plans to introduce a new fleet of driver-only-operated trains from 2020