Network Rail has renewed the rail track between the stations at Hemel Hempstead and Berkhamsted following a programme of recent upgrades.
The rail firm took the opportunity to carry out extensive track and drainage work on sections of the West Coast main line over Bank Holiday August weekend.
The line is one of the most intensely used pieces of railway in Europe.
James Dean, director of asset management for the London North Western route at Network Rail, said: "I would like to thank passengers for their understanding over this August bank holiday whilst work took place across the network. The upgrade, maintenance and renewal work forms part of our ongoing Railway Upgrade Plan which is providing customers with a better railway.
“There is never a good time to carry out work that affects services but we worked closely with the train operators for it to cause the least amount of disruption. I would like to thank passengers for their understanding while we completed these vital upgrades.”
London Midland's head of West Coast services, Steve Helfet, said: "We are so grateful to all our customers who heeded the message not to travel during the work unless absolutely necessary."