Members of the Rail, Maritime and Transport union on Arriva Rail North launched a three-day walkout, while guards on Merseyrail will strike today and on Monday.
Services on both operators were hit, with passengers urged to check before they travel.
RMT general secretary Mick Cash said: “Once again RMT members are standing firm in the fight for safe and accessible rail services.
“The action in the two separate disputes on Northern and Merseyrail is being solidly supported across all routes in the battle for safe railways for all.
“Despite relentless campaigns of threats and intimidation RMT members are out on the picket lines this morning putting the basic principle of rail safety before the profits of the rogue rail operating companies.
“Both Northern and Merseyrail should get out of the bunker, stop doing the bidding of their political masters and start talking seriously with the union about a safe and secure future that guarantees the role of the guard on their trains.”
Most Merseyrail services are planned to run every half hour from around 7am to 7pm, although will not stop at some stations.
Northern is planning on running more than 40% of services on revised timetables between 7am and 7pm today and Monday, and between around 9am and 5pm on Sunday.
Jan Chaudhry-van der Velde, managing director at Merseyrail said: “The Merseyrail team has put together the best possible timetable that we can to provide a limited train service on both strike days. I would urge all our customers to check before they travel.”
Sharon Keith, regional director at Northern said: “We are doing everything we can to keep our customers on the move during the three days of industrial action affecting our network.
“We will be running more than 40% of our usual services, but those trains that do run will be on an amended timetable and are likely to be extremely busy.
“We ask our customers to plan carefully, allow extra time for travel, and to consider whether their journeys are necessary.”
RMT members on Southern Rail will strike for 24 hours on Monday in a long-running dispute over the role of conductors.