At Birmingham New Street, trains will finish much earlier than normal and only ten trains per hour will depart from the station, compared to an average of 40 on a non-strike weekday.
Birmingham New Street's last trains will leave between 3pm and 5pm with final arrivals by 6.30pm.
The station will close completely at 10pm on Wednesday evening.
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Meanwhile, at London Euston, only five trains per hour will depart from the station, compared to an average of 17 on a normal day.
Many of its last trains will leave between 3pm and 5pm and arrive at their final destinations by 6.30pm.
For both London Euston and Birmingham New Street, trains which do run will start later than 7.30am.
A Network Rail spokesman said: "People are once again being advised to only travel by rail if necessary tomorrow because of the continuing staff walkouts by RMT union members.
Leighton Buzzard passengers can also check with their train operator via: https://www.londonnorthwesternrailway.co.uk/travel-information/journey-planning/service-disruptions/industrial-action
The spokesman added: "Across the country, thousands of specially-trained and fully qualified back-up staff will again step in during the walkout to keep vital services running for those who need them.
"But with only a fifth of services running, on around just half of the network, passengers are asked to only travel by train if they must.
"Passengers should also expect disruption on the morning of Thursday, July 28, with a later start to services as employees return to duties."
Separately, train drivers' union Aslef has called strike action across seven train operators on Saturday, July 30, that will "significantly disrupt some routes", so passengers are asked to check their journeys.
If commuters with an advance, off-peak or anytime ticket are affected by the strike on July 27 or 30, they can use their ticket either on the day before the date on the ticket, or up to and including Tuesday, August 2.
For more information, visit: https://www.nationalrail.co.uk/