East Midlands and Thameslink train services have been hit by major delays because of "damage to overhead wires" at Luton, National Rail said.
Severe disruption is expected until the end of service despite the line between Bedford and St Albans reopening.
Thameslink said that power to the overhead wires had been restored but that further signalling problems were still affecting services.
They advised customers to use alternative routes as a "very limited" service was in place.
East Midlands warned passengers that the disruption could continue into Wednesday morning but said they were working with Network Rail to fix the problem.
The damage has caused disruption on East Midlands trains between Corby, Nottingham, Sheffield and London St Pancras, while Thameslink services between Bedford, Luton and Three Bridges, Sutton, Sevenoaks and Gatwick Airport are also delay-hit.
National Rail said: "Trains are currently unable to run between Bedford and St Albans and as a result trains to and from London St Pancras International may be amended, cancelled or delayed by up to 90 minutes.
"Although Network Rail engineers are attending the problem, we expect disruption to continue until the end of service."
East Midland commuters can use tickets to travel London Midland between London Euston and Northampton, CrossCountry between Derby, Leicester, Birmingham and Peterborough as well as Virgin Trains on "reasonable routes".
A replacement bus is running between Kettering and Corby as well as between Wellingborough and Northampton.
The overground, underground, London buses and other train services can also be used with Thameslink tickets on reasonable routes.
Auxiliary road transport is also shuttling between St Albans and Bedford.
A Network Rail spokesman said: "Due to a piece of cable coming into contact with the overhead electric lines at Luton station we are currently unable to run trains through the area.
"We are working to repair the problem and get trains running again as soon as possible with Network Rail teams currently on site.
"We apologise for any inconvenience this may cause and advise that passengers travelling through Luton this evening check with their train operator before they travel."
Edmund King, AA president, said he had been caught up in the "chaos" at St Pancras after travelling by train from Brussels to London.
He said that "hundreds" of people were being "held back behind the barriers with no information although an official managed to shout loud enough to try to clear an exit but didn't bother telling us what was going on".
"The sensible people clubbed together to pay £45 for a taxi to St Albans. Others of us struggled onto a train and were squashed like sardines amidst sneezing, sniffing, sad passengers. Not a pleasant experience where a 20 minute journey took more than an hour."