A month of engineering works begins this weekend, affecting rail services to and from Milton Keynes.
Works will take place on the weekends of August 9-10, 16-17, and 23-25.
On most of these days trains will run as far south as Hemel Hemstead, with the rest of the route replaced by buses running every 30 minutes.
On Saturday, August 24, trains will not run south of Milton Keynes, and will be replaced by buses from the city.
Passengers are also advised to check train times on Monday, August 11, and Monday, August 18, as there will be revised services in operation for the majority of the day.
Terry Oliver, London Midland’s head of West Coast services, said: “If passengers can avoid travelling into and out of the City on these days, then I would advise them to do so. The limited number of trains running means that it will be extremely busy.
“London Midland will have staff at stations along the route over the next three weekends to provide assistance and travel advice.
“We’d like to thank our passengers in advance for their patience and co-operation.”
This is the second phase of closures as part of Network Rail’s £81m improvement plan to replace track, signalling equipment and overhead power lines in the Watford area.
The first closure took place in May 2014, when similar alternative travel arrangements were put in place.