Members of the Rail, Maritime and Transport union (RMT) at Network Rail and 14 train operators will walk out on August 18 and 20. It means ScotRail services are likely to be crippled on days during the Edinburgh festivals as signallers walk out.
The union previously announced a strike for July 27, the day before the Commonwealth Games opens in Birmingham.
RMT general secretary Mick Lynch said: “The rail industry and the Government need to understand that this dispute will not simply vanish.
“They need to get serious about providing an offer on pay which helps deal with the cost-of-living crisis, job security for our members and provides good conditions at work.
“Recent proposals from Network Rail fell well short on pay and on safety around maintenance work.
“And the train operating companies have not even made us a pay offer in recent negotiations.
“Now Grant Shapps (Transport Secretary) has abandoned his forlorn hopes for the job of prime minister, he can now get back to his day job and help sort this mess out.
“We remain open for talks, but we will continue our campaign until we reach a negotiated settlement.”
Andrew Haines, chief executive of Network Rail, said: “By announcing even more strike dates, the RMT has dropped any pretence that this is about reaching a deal.
“It’s clear the best interests of passengers and our staff are taking second place to the union’s bosses’ political campaign.”
The drivers’ union Aslef earlier announced its members at eight train companies will strike on July 30 in the same dispute, which will impact cross-border LNER services but not ScotRail.
A spokesperson for the Rail Delivery Group said: “This is a hugely disappointing announcement from the RMT’s leadership which will upset passengers’ summer plans, undermine struggling businesses and upend the industry’s recovery.
“We want to give our people an increase in pay, but asking taxpayers to shoulder more of the burden when they have already contributed £600 per household during the pandemic, or expecting passengers to fund it by paying more for their tickets, isn’t fair or sustainable.
“Instead, we have a responsibility to make changes to long-outdated working practices so we can adapt to post-Covid travel patterns, bring our railway up to date and give our passengers a more punctual and reliable service.
“Rather than staging counterproductive strikes, we ask the RMT’s leadership to continue talking so we can come to a deal that works for our people, our passengers and for taxpayers.”
A Department for Transport spokesperson said: “As the Secretary of State said only yesterday, recent talks have merely been for show while the RMT have been planning how best to create further misery for passengers across the UK. These strikes will be a kick in the teeth for millions of people who stumped up £600 per household to keep the railway running throughout the pandemic.
”The rail industry has to modernise and be brought into the 21st century for the benefit of passengers and staff alike. There’s no getting around that and we’re extremely disappointed to see more strikes announced.”