Extreme heat can cause rails to expand and buckle so when temperatures reach certain thresholds, trains must run more slowly to keep services safe.
Passengers are advised to check their journeys before travelling on Tuesday and to only travel on Wednesday and Thursday if their journey is essential due to the red Met Office warning for extreme heat.
Amended timetables will be uploaded to journey planners overnight. Routes with a reduced service level from Tuesday include:
Jonny Wiseman, WMR customer experience and commercial director, said: "With extremely high temperatures forecast over the coming days we will be running a significantly reduced service across our network for safety reasons.
"We advise our customers to check before travelling on Tuesday and to only travel on Wednesday and Thursday if their journey is essential.”
