Major disruption is expected at London Euston for a third day after a kilometre of cable was damaged.
Overhead power wires were damaged near Wembley on Saturday causing trains to be cancelled or delayed over the weekend.
National Rail said engineers had been working overnight to make repairs but a “significant amount of work” was still required.
Disruption is expected to last until the end of the day.
Network Rail said it had completed work on three of the four lines affected but some routes still face problems:
- Avanti West Coast between London Euston and Birmingham New Street / Liverpool Lime Street / Holyhead / Manchester Piccadilly / Glasgow Central / Edinburgh
- London Northwestern Railway between London Euston and Northampton / Milton Keynes Central / Stafford / Crewe
- Southern Railway services between East Croydon and Milton Keynes Central
Avanti West Coast is running a reduced service during the day and have warned that the trains which are running will be extremely busy.
The firm has advised passengers their tickets will still be valid on Tuesday.
London Northwestern Railway services are expected to be particularly disrupted from Tring, Berkhamsted, Hemel Hempstead, Apsley, Kings Langley, Watford Junction, Bushey and Harrow and Wealdstone.
A shuttle bus is running between Hemel Hempstead, Apsley, Kings Langley and Watford Junction, the company said.
Southern Railway said a shuttle service had been put in place between Clapham Junction and Shepherds Bush and will run until the end of the day.
London Overground trains between Euston and Watford Junction and Caledonian Sleeper services are no longer affected by the problem.