Trains services are due to be suspended from Watford Junction, Bushey and Kings Langley stations for three weekends in August.
Rail improvement works next month mean passengers will have to use replacement buses as London Midland won’t be operating a service between London Euston and Hemel Hempstead.
Works are planned for Saturday, August 9, to Monday, August 11; Saturday, August 16, to Monday, August 18; and Saturday, August 23, to Tuesday, August 26.
Rebecca Freeman, London Midland’s project manager for the Watford Area Renewals Programme, said: "We know that the closures will affect many people’s travel plans and we’d like to thank our customers in advance for their patience and cooperation.
"During the closures, journey times will be extended and will involve various changes. I urge passengers who need to travel to check in advance and plan their journey."
London Midland has also advised passengers to consider making alternative work or travel arrangements on Monday, August 11, and Monday, August 18, as there will be a reduced train service into London on these days.
Ms Freeman continued: "Those two days provide an opportunity to take advantage of flexible working or annual leave, especially as it’s the school holidays. We wanted to let our customers know about the reduced service now, so that they have plenty of time to make arrangements."
Network Rail is carrying out the significant upgrade work as part of an £81million investment to improve reliability and punctuality on the West Coast Mainline.
Terry Strickland, area director for Network Rail, said: "This scheme represents significant investment in the southern end of the West Coast Main Line to improve punctuality and make the railway more reliable.
"This line has seen tremendous growth in traffic and passengers over the last five years with the section at Watford being one of the most intensively used, high-speed sections of track in the UK. This work is absolutely essential to maintain reliable train services for the thousands of passengers who travel on this route every day."
Online journey planning information has been amended to reflect the changes that will be in place. Visit www.londonmidland.com/Watford for more information on alternative travel arrangements. For further details about the improvement works programme visit www.nationalrail.co.uk/Watford.