Cross-channel Eurostar train services remained suspended all weekend after the cold weather was blamed for the breakdown of four trains on Friday, trapping more than 2,000 people for hours.
On Sunday Eurostar engineers began running empty test trains to begin to try and work out why cold air caused four trains to break down inside the tunnel on Friday.
“Following the disruption to its services over the last 36 hours, Eurostar does not want to cause its passengers any further disruption and will be conducting a programme of ‘test-trains’ which will not carry passengers today to better understand the problems that have been occurring,” the company said in a statement.
“We are committed to restoring our services as soon as possible but our key priority is the safety and comfort of our customers.”
The company is fully reimbursing passengers and paying travellers for “reasonable” out of pocket expenses.
More than 2,000 people were stuck after four Eurostar trains broke down, while roads and air travel were also disrupted by the cold snap.
It is thought the trains failed as they left the cold air in northern France and entered the warmer air inside the tunnel, Eurostar spokesman Bram Smets said.
People were stuck in France and Folkestone, while hundreds gathered at London’s St Pancras terminal.
The tunnel closure added to travel problems caused by heavy snow in Kent and police have brought in Operation Stack - which allows lorries to park along sections of the M20 - to ease congestion near the terminal.