Up to 2,000 rail workers represented by the RMT union started a 24-hour walkout across the Southern, Merseyrail and Arriva Trains North franchises on Monday (13 March).
The strikes are over the future role of rail guards, with the companies moving towards driver-only operations (DOOs).
RMT general secretary Mick Cash described the industrial action as "rock solid" in a statement.
"RMT's action on Arriva Rail North this morning is absolutely rock solid and determined as our members fight for passenger safety and the retention of the guard on the Northern Rail services. Our pickets are out across the franchise and the response has been fantastic," he said.
"Arriva Rail North should listen to their staff, listen to the public and recognise that there is no case whatsoever for axing the guard from their trains.
"This action could have been avoided if Arriva hadn't back pedalled from earlier pledges to retain the guard. It is now down to the company to get that pledge back on the table and engage with the union in talks over a safe and sustainable future built around the guarantee of a guard on the trains."
The walkouts are expected to hit around 270,000 commuters. Merseyrail, Arriva Trains North and Southern are running reduced services.
"It is disappointing that the RMT are disrupting passengers across the country," said rail minister Paul Maynard said. "These strikes are unnecessary and we call on the RMT to return to talks and help to deliver the high quality rail services that passengers deserve."