The union says 40,000 railway workers will walk out on June 21, 23, and 25 in a dispute over "safety critical" jobs and pay.
London Northwestern Railway says it expects "significant" disruption to its services between London, Birmingham and the North West via Northampton and Long Buckby.
And a spokesperson for East Midlands Railway, which operates trains from Corby, Kettering and Wellingborough, warned: “This will have an impact on the level of service we are able to operate.
“EMR has extensive contingency plans in place and will run as many services as possible.”
The walkout is also likely to affect services on days before, after and between the announced strike dates.
Both companies say they will publish more information in the coming days.
Passengers already booked are being offered opportunity to swap their tickets for other days or full refunds.
RMT claims Network Rail plans to cut up to 2,500 jobs as part of a £2 billion reduction in spending while "fat cat” rail bosses have been paid millions during the Covid-19 pandemic.
Union leader Mick Lynch claims says operating companies nationally have subjected their staff to multi-year pay freezes and plan to cut thousands of jobs which will make the railways unsafe.
He added: “Railway workers have been treated appallingly and despite our best efforts in negotiations, the rail industry with the support of the government has failed to take their concerns seriously.”
The Rail Delivery Group, which represents 13 train operating companies, says reform is necessary to secure a long-term future for the railway.
Chair, Steve Montgomery, said: “We urge the RMT’s leadership to call off needless and damaging strikes and continue to work with us to ensure a fair deal for our people and for the taxpayer.
"No one wins in the event of a strike. Staff lose pay, the industry loses vital revenue making it harder to afford pay increases, and passengers and businesses are disrupted.
“While we will keep as many services running as possible, sadly if this action goes ahead, significant disruption will be inevitable.”