Arriva Rail North staff will walk out for three days from 8 July, while Merseyrail staff will strike on 8, 10 and 23 July, the final day of The Open.
Workers at Southern also plan to walk out on 10 July.
The RMT is in dispute with the rail companies over driver-only-operated trains which they say would be unsafe and lead to widespread job losses.
The strike will also clash with the British Style Collective, the Clothes Show's big event in Liverpool from 7 - 9 July.
RMT General Secretary Mick Cash said any responsibility for disruption to services lay with the companies and their refusal to enter meaningful talks with the union.
"It is that flagrant disregard for the safety issues at the heart of the dispute which leaves us with no option but to press on with the campaign of strike action," he added.
However, Richard Allen, deputy managing director for Arriva-run Northern, said he was "deeply disappointed and hugely frustrated" by the announcement of further action.
"Only this week we wrote to RMT again, urging them to engage in meaningful modernisation talks with us.
"We have been clear that for all our conductors we are prepared to guarantee jobs and current pay, and continue with annual pay reviews, if we can reach agreement with RMT," Mr Allen said.
He added fresh industrial action would not solve the dispute.
Merseyrail's managing director Jan Chaudhry-van der Velde criticised the timing of the strike which he said was designed to "cause maximum disruption to people attending the last day of the Open Golf at Royal Birkdale".
Mr van der Velde claimed the company "brought several new initiatives to the table in the last round of talks, the RMT is unprepared to work with us to find middle ground".
"While we, as always, will do our best to minimise disruption, our passengers should not be made to suffer at the hands of the RMT, and the Liverpool City Region should not be caught in the cross-fire of a national dispute directed by unions in London."
The latest strike announcement comes as a long-awaited report into the troubled Southern rail franchise has blamed the unions for widespread disruption over the last year.

Kaleem Ali, 18, approached one of the women who gave evidence in the trial of his father Qurban and five others.
The victim said she had been left "shaking and in fear of her safety".
Ali, of Clough Road, Masborough, received a three-year restraining order at Rotherham Magistrates' Court.
He was also ordered to pay Â£360 towards the cost of the prosecution and given 12-week curfew requirement.
Qurban Ali, 53, was one of six people jailed in February for the "systematic abuse" of  15 girls. He was convicted of conspiracy to rape and jailed for 10 years.
Prosecutor Mark Hughes told the court Kaleem Ali approached the woman in March 2015 as she was in her car with her children.
He said Ali addressed the woman and one child by name and appeared to start taking photographs of them on one mobile phone while using another to make a call.
Reading from the victim's police statement police, he said: "I truly believe this male wanted to scare me and it worked. I was crying and shaking. I was scared he was calling for other people to join him. I had my children with me and I feared for my safety."
Mr Hughes said Ali had accepted seeing the witness but denied taking photographs and denied that his actions were intended to intimidate.
Ali's lawyer, Steve Smith, said his client had learning difficulties which left him unable to understand how people perceived him or his actions.
He said Ali had approached the woman as a result of a "chance meeting".
Train passengers across England are to be hit by further strike action, RMT officials have said.
[NEXT_CONCEPT]
The son of a man jailed as part of a gang who groomed and abused teenage girls in Rotherham has been given a 12-month community order after being found guilty of witness intimidation.