Rail passengers between London St Pancras and Nottingham and Sheffield have been urged not to travel as disruption caused by hot weather enters a third day, and conductors strike.
Engineers are still trying to fix overhead line equipment near West Hampstead, north-west London.
The damage was caused by the heat on Thursday – when the UK recorded its second hottest day ever.
Train companies told customers to expect a reduced service until Monday.
East Midlands Trains said it was running a heavily amended service on the route.
Meanwhile, a second successive Saturday walkout by members of the RMT union over pay and conditions is also taking place, affecting local journeys on East Midlands Trains.
Some services have been replaced by revised timetables and coach services.
The power line repairs mean tickets on East Midlands Trains and Thameslink – which runs services in and around London – are being accepted on alternative routes.
National Rail advised those travelling between London and Sheffield to use the line linking London King’s Cross with Doncaster.
It said passengers in Nottingham should travel via Grantham to King’s Cross, while passengers in Leicester should travel to London Euston via Nuneaton.
Thameslink told customers Network Rail has been “unable to completely fix” the damaged wires between St Pancras and Elstree.
A reduced service is expected to run for the rest of Saturday, and Thameslink says it has requested extra rail replacement buses for Sunday.
Eurostar – which runs services out of St Pancras International – was not reporting problems on its route to the Continent.
Commuters faced widespread disruption as a result of heat-related incidents on the railway while flights were also disrupted by storms across Europe.
Jake Kelly, East Midlands Trains’ managing director, apologised to passengers, saying the disruption would continue into the weekend.
He said the company was working “very closely” with Network Rail engineers to repair the equipment and fully reopen the railway.
“Our advice for customers is to avoid travelling on this route wherever possible and make alternative arrangements,” he added.
Network Rail warned trains which do run are expected to be “incredibly busy”.
Sue Grenwood, who was travelling to St Pancras from Derby, thanked staff for making the best of things.
She tweeted: “All kinds of delays and difficulties going on but Derby team found us a replacement train to St Pancras, and skeleton crew, and the most upbeat staff.”
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