Network Rail said new track, sleepers and ballast would be installed near Redruth, with track replaced near Camborne, and track equipment also upgraded near Penzance and St Erth.
Lee Goodson, Great Western Railway station manager for west Cornwall, said: “During these dates no trains can stop at Redruth, Camborne, Hayle, St Erth or Penzance or at any of the stations on the St Ives Bay Line.
“Trains will still be able to run at Truro for Falmouth Docks as well as trains for Exeter St Davids or London Paddington. But CrossCountry trains will not be operating between Truro and Plymouth.”
Mr Goodson added: “It’s important that customers are aware these alternative travel arrangements will make journey times much longer.”