The union announced the news on Monday, ahead of separate strike action beginning on Wednesday July 27.
The Transport Salaried Staffs Association (TSSA) said members at seven train companies will walk out on August 18 and 20.
They are the same days as the Rail, Maritime and Transport union is striking against Network Rail and 14 train operators.
The TSSA said thousands of its members – including station staff, operational, maintenance, supervisory and management staff – will take part in industrial action.
Trains across the country will be hit by industrial action over the next month
Andrew Milligan
Avanti West Coast – and several other providers – will be hit
Matthew Pover/Avanti West Coast
Avanti West Coast, c2c, East Midlands Railway, CrossCountry, Great Western Railway, LNER, and Southeastern services will all be hit.
Action that stops short of a fully-fledged strike will hit West Midlands Trains, Northern, Greater Anglia, TransPennine Express and Southeastern services.
The TSSA has held off on serving notice for industrial action, to hold 11th-hour talks with Network Rail over pay, job security and conditions.
There is still time to serve notice for action on August 18 and 20 if these talks are unsuccessful.
Manuel Cortes, TSSA general secretary, said: “This is a momentous day for our members. The Tories’ cost-of-living crisis is the worst in living memory. Essential items like food, energy and clothing costs are going through the roof yet the Government has chosen to pick a political fight with rail workers.
“Most of our members are going into a third or fourth year of pay freezes, seeing their real take home pay decrease. For many rail workers in our union this is the first time they have been directly involved in an industrial dispute.
“We do not take strike action lightly, but enough is enough. The Conservative government is the clear block to a deal for rail workers.
“Transport Secretary Grant Shapps must either personally come to the table or empower train operators to reach a deal on pay, job security and conditions.
“Instead of wanting to resolve this dispute, we now see proposals for hundreds of ticket office closures and widespread job cuts across our railways.
“We’ve been warning of a Summer of discontent across our railways for months, and sadly it is an ever-closer reality.”
Responding to the strike announcement, a Department for Transport spokesperson said: "It’s hugely disappointing that, rather than commit to serious dialogue with the industry, the TSSA is seeking to cause further misery to passengers by cynically coordinating strikes to cause maximum disruption to the rail network.
“Our railway is in desperate need of modernisation to make it work better for passengers and be financially sustainable for the long term.
“The only thing more strikes will do, however, is wreak further havoc on the very people unions claim to stand up for – people who, on average, stumped up £600 per household to keep our railways running throughout the pandemic while ensuring not a single worker lost their job.
“We urge the TSSA to reconsider and work with its employers, not against them, to agree a new way forward.”