Commuters heading into and out of Clapham Junction and Euston could face delays for the rest of the week.
A landslip near Leighton Buzzard has disrupted Virgin Trains, Southern and London Midland services.
National Rail said the problems were expected to last until 6am on Friday 5 February.
Virgin and London Midland trains between Milton Keynes Central and London Euston were being delayed by up to 15 minutes on Monday.
Southern services between Milton Keynes Central and Watford Junction were suspended.
Trains between Euston and Milton Keynes will see revised timetables for the rest of the week.
— Network Rail (@networkrail) February 1, 2016
London Midland passengers have been told to use the Overground or Underground on any reasonable route.
Commuters on Southern trains have been advised to use London Midland trains between Milton Keynes and Euston, and the Underground between Euston and Victoria.
Network Rail engineers have started work on replacing 2,200 tonnes of material on the damaged railway bank in Bedfordshire.
Area director Mark Killick said he recognised the disruption for passengers and would keep them updated on progress.
“We are aiming to complete this work by Friday and can’t be any more definitive at this point,” he said.